PER CURIAM.

Petitioner appeals as of right from the Tax Tribunal's order declaring it liable for $81,470.89 in back taxes under the use tax act, MCL 205.91 et seq.; MSA 7.555(1) et seq. We find no error and affirm.

We first note the standard for reviewing Tax Tribunal decisions involving application of the use tax. The use tax is not a property tax, Banner Laundering Co v State Bd of Tax Administration, 297 Mich. 419, 427; 298 N.W. 73 (1941). Accordingly, the present case is not governed by the standard of review set forth in the last sentence of Const 1963, art 6, § 28, which applies solely to cases involving application of the property tax. Instead, the more general provisions of Const 1963, art 6, § 28 apply, as follows:

"[R]eview shall include * * * the determination whether * * * final decisions * * * are authorized by law; and, in cases where a hearing is required, whether the same are supported by competent, material and substantial evidence on the whole record."

Applying the foregoing standard of review, we find that the tribunal's decision must be upheld, as it is supported by competent, material and substantial evidence.

What petitioner challenges on appeal is the tribunal's finding that petitioner's purchases of exchange services from Michigan Bell are "intrastate" within the meaning of MCL 205.93a; MSA 7.555(3a). This finding was based upon evidence of record indicating that the service which petitioner purchases is necessarily located and delivered solely in Michigan.

A schematic diagram submitted by petitioner at the hearing reveals the essentially local character of the exchange services. The diagram shows that petitioner's intercity network is distinct from the local exchange facility which transmits messages to and from local subscribers. A message (or more precisely, the electronic impulse which carries any message) passes over Michigan Bell's lines — lines which are necessarily located within this state — before petitioner ever takes control or dominion over the message. Similarly, the message passes through a local exchange facility before entering petitioner's interstate network.

Petitioner makes much of the fact that it does operate an interstate network and that even its Michigan customers making calls within this state depend upon certain out-of-state elements of that network in order to complete their calls. From this fact, petitioner reasons that it provides an interstate communication service and that, when it purchased certain exchange services from Michigan Bell, petitioner was merely purchasing an integral part of the interstate communication service which it provides to its customers. We believe that petitioner's argument overlooks the crucial distinction between the service which it provides to its own customers and that which it has purchased from Michigan Bell. Respondent does not propose to tax revenue from interstate calls made by petitioner's Michigan customers. Instead, respondent proposed only to tax the amounts which petitioner has paid to Michigan Bell in order to facilitate its customers' calls. The fact that its customers make calls using parts of the interstate network does not change the fact that petitioner has purchased an exchange service which was in all respects provided and located in Michigan.

In urging that its interstate network must be treated as inseparable from Michigan Bell's local exchange services, petitioner misplaces reliance upon several federal cases, United States v South-Western Cable Co, 392 U.S. 157; 88 S Ct 1994; 20 L Ed 2d 1001 (1968); Ward v Northern Ohio Telephone Co, 300 F.2d 816 (CA 6, 1962), cert den 371 U.S. 820; 83 S Ct 37; 9 L Ed 2d 61 (1962); Idaho Microwave, Inc v Federal Communications Comm, 122 US App DC 253; 352 F.2d 729 (1965). These cases held only that companies which limit their services to a single state are still part of an interstate broadcasting or communications industry and are thus still subject to FCC regulation. The cases do not hold that such companies are providing only interstate services, nor do they preclude the possibility that such companies could also be providing certain intrastate services which might be appropriate subjects of state regulation or taxation. See Ward, supra. Michigan courts have long recognized that a business can be both interstate and intrastate in character, J B Simpson, Inc v State Bd of Tax Administration, 297 Mich. 403, 409; 298 N.W. 81 (1941). Similarly, the United States Supreme Court had identified a separate intrastate aspect or phase of an otherwise interstate industry or activity, United Air Lines v Mahin, 410 U.S. 623; 93 S Ct 1186; 35 L Ed 2d 545 (1973); Complete Auto Transit, Inc v Brady, 430 U.S. 274; 97 S Ct 1076; 51 L Ed 2d 326 (1977). Accord, Bob-Lo Co v Dep't of Treasury, 112 Mich. App. 231, 239-242; 315 N.W.2d 902 (1982). In the present case, the local exchange services which plaintiff purchased constitute an activity which is intrastate in character even though petitioner in turn provides interstate services to its customers. We conclude that the Tax Tribunal acted properly in characterizing Michigan Bell's exchange services as a taxable intrastate aspect of petitioner's business.

Petitioner next urges that the tax on petitioner's use of local lines violates the Commerce Clause, United States Const, art 1, § 8. In order for a tax to be constitutional, the following four factors must be present: (1) the taxed activity must have a substantial nexus to the taxing state; (2) the tax must be fairly apportioned; (3) the tax must not discriminate against interstate commerce; and (4) it must be fairly related to services provided by the state. Complete Auto Transit, Inc v Brady, supra, pp 277-287. Petitioner does not contend that the tax is unfairly apportioned or that it is not fairly related to services provided by this state. Instead, petitioner focuses upon the first and third factors, supra, urging that each is absent and that, as a result, the tax violates the Commerce Clause. We disagree.

The tax does have a sufficient nexus to Michigan, since it is applied to local exchange services provided by a corporation which can operate only in Michigan. As in Complete Auto Transit, supra, the intrastate link or aspect in the taxed company's interstate network is identifiable and a proper subject of state taxation. Petitioner's assertions to the contrary, there is a taxable event or transaction which takes place in Michigan. Petitioner has to purchase access to Michigan Bell's local communications network in order to complete the transmission of its messages. The point of access or transfer necessarily takes place within Michigan; petitioner's own network reaches into this state to meet that of Michigan Bell at that point of access. Contrast Michigan Wisconsin Pipe Line Co v Michigan, 58 Mich. App. 318; 227 N.W.2d 334 (1975), relied on by petitioner, where the gas in question never left the taxpayer's own pipeline. There was no transaction between the taxpayer and any other company, and the taxpayer neither purchased nor consumed any Michigan company's services.

The taxpayer in Michigan Wisconsin Pipe Line Co was merely using its pipelines to pass its product through Michigan, without any taxable exchange of goods or services. The present case does involve an identifiable exchange of services within Michigan borders and involves no attempt to tax petitioner for merely passing its message through this state. Cf. Helson v Kentucky, 279 U.S. 245; 49 S Ct 279; 73 L Ed 683 (1929), Michigan Wisconsin Pipe Line Co, supra. The Supreme Court has upheld tolls for the use of facilities located within one state, even where those facilities form a link in a network of interstate commerce. United Air Lines, supra; Evansville-Vanderburgh Airport Authority Dist v Delta Airlines, Inc, 405 U.S. 707; 92 S Ct 1349; 31 L Ed 2d 620 (1972). Users of facilities in the taxing state may properly be required to pay their fair share of the cost of providing, maintaining and protecting those facilities. United Air Lines, supra, p 630, fn 7.

The use tax in the present case does not require petitioner to pay anything more than its nondiscriminatory share of Michigan's cost in protecting or maintaining Michigan Bell's local exchange service facilities. Accordingly, there is no merit to petitioner's alternative claim that it faces the possibility of multiple, discriminatory taxation. The tax is solely upon the privilege of consuming services provided within Michigan; there is no possibility that the tax can be duplicated by another state. Any other state's taxation of exchange services would be limited to amounts spent on separate exchange services provided within that other state. There is no showing of any attempt by other states to tax services which plaintiff purchased and received in Michigan. In short, there has been no showing that Michigan's tax, when combined with that of other states, would result in multiple taxation of the activity in question, namely, the local exchange services provided to plaintiff by Michigan Bell.

Petitioner next urges that the use tax would interfere with federal regulation of interstate communications, in violation of the Supremacy Clause, US Const, art 6, § 2. We find no merit in this argument. Petitioner has failed to meet its burden of showing how local taxation actually interferes with any federal agency's regulations. The cases cited by petitioner, Maryland v Louisiana, 451 U.S. 725; 101 S Ct 2114; 68 L Ed 2d 576 (1981), and Northern Natural Gas Co v Kansas State Corporation Comm, 372 U.S. 84; 83 S Ct 646; 9 L Ed 2d 601 (1963), are distinguishable; each tax statute contained specific directives which purported to require a federal agency to allocate costs in a certain manner. Taxes on interstate commerce are only found void as inconsistent with federal regulation where the taxes are linked to specific regulatory directives which would demonstrably impede congressional objectives. See, for example, Bell Telephone Co of Pennsylvania v Federal Communications Comm, 503 F.2d 1250 (CA 3, 1974); North Carolina Utilities Comm v Federal Communications Comm, 537 F.2d 787 (CA 4, 1976).

In the present case, Michigan's use tax act contains no directives which would require a federal agency to allocate costs in a particular manner. Its sole effect would be to increase somewhat the cost of petitioner's doing business in this state. It is true that the Federal Communications Commission will have to take into account the increased cost of doing business in determining what constitutes a fair and reasonable rate for its regulated Michigan business. However, so causing a federal agency to consider certain costs of doing business is far different from setting forth directives which leave it no choice but to allocate costs in a particular manner. Under plaintiff's argument, no enterprise subject to federal rate regulation could ever be taxed by a state once initial rates have been set, because state taxes are elements of the enterprise's cost of doing business, and any increase in taxation would "compel" a reconsideration of the rate structure. Since plaintiff has cited no authority for such a sweeping proposition, we decline to adopt it here. Given the foregoing, we conclude that there has been no showing of any Supremacy Clause violation.

Petitioner's final argument on appeal is that respondent's disparate treatment of petitioner and ATT amounted to a denial of equal protection. We find no basis for reversal. Petitioner must show that respondent has failed to tax similarly situated enterprises and that its failure to do so was intentional and knowing, rather than mistaken or the result of inadvertence. Sunday Lake Iron Co v Wakefield Twp, 247 U.S. 350; 38 S Ct 495; 62 L Ed 1154 (1918); Titus v State Tax Comm, 374 Mich. 476, 487; 132 N.W.2d 647 (1965) (ADAMS, J., dissenting). The Tax Tribunal found that respondent's classification attempted to distinguish those who purchase local telephone exchange services from those who do not. The tribunal went on to find that respondent "properly included [petitioner] in that classification of taxpayers" but that respondent made an "error" in its "failure to investigate the compensation scheme between ATT Long Lines and Michigan Bell". The tribunal found that respondent had "missed a potential taxpayer" because of respondent's failure to "uncover [sic] that ATT Long Lines may also be included in the purchasing of service category". The tribunal's characterization of respondent's conduct as mistaken, as opposed to intentional, is a factual finding which this Court must uphold if supported by competent, material and substantial evidence.

We believe that the evidence did in fact support a finding that respondent's failure to tax ATT was a mistake, rather than deliberate discrimination. The tariffs or rates governing petitioner's relationship with Michigan Bell were not identical to those governing ATT. Until the tribunal held its hearing below, respondent could well have believed that ATT was merely sharing revenues with its subsidiary, Michigan Bell. Similarly, respondent could also have formed a mistaken but innocent belief that in ATT's case, there was in fact a unitary "interstate" network, whose indivisible character arose from ATT's ownership of the entire enterprise. Applying this reasoning, respondent may have erroneously, yet without discriminatory intent, concluded that petitioner's use of Michigan Bell's exchange services was taxable, while ATT's comparable use of local exchange facilities was not.

Petitioner has demonstrated the error of this reasoning. As noted above, any company's use of Michigan Bell's local exchange services is an intrastate aspect of that company's otherwise interstate business, properly subject to state taxation. Thus, ATT's use of those Michigan Bell services is just as taxable as plaintiff's. However, petitioner has not demonstrated that respondent's error in omitting taxing ATT was knowing or intentional. Accordingly, there has been no showing that respondent's failure to tax ATT amounted to an equal protection violation under the standards announced in Sunday Lake Iron, supra, and Titus, supra.

In concluding our review of this issue, we find one caveat appropriate. Arguably, continued failure to tax similarly situated enterprises when the taxing authority is aware of a valid equal protection challenge such as the present one can itself evidence an ongoing equal protection violation. See Wakefield, supra, pp 352-353. Accordingly, any continued failure to tax ATT could result in a violation of petitioner's right to equal protection, particularly for taxes applicable to periods following the release of this opinion. In order to avoid a finding that the imposition of future taxes on petitioner would violate equal protection, respondent should take affirmative steps toward imposing use taxes upon ATT's analogous purchase of local exchange services. Since ATT's tax liability is not at issue here, our comment as to this matter is limited to the foregoing caveat concerning the validity of potential future use taxes upon petitioner. Our comment should not be construed as a binding order to impose similar taxes upon ATT. Respondent may ultimately decide to adopt the alternative nondiscriminatory policy of taxing neither ATT nor petitioner for their purchase of local exchange services.

Petitioner has not sought any remedy for respondent's failure to tax ATT during the period between the hearing date below, when respondent first became aware of its error in failing to tax ATT, and the issuance of this opinion. Accordingly, we decline to address the question of whether an ongoing equal protection violation has occurred since the hearing date.

The foregoing compels our conclusion that the use tax assessed by respondent for the period preceding the hearing, April 1, 1974, through March 31, 1978, was valid.

Affirmed.