29 C.F.R. § 553.105
An agreement between two or more States, political subdivisions, or interstate governmental agencies for mutual aid does not change the otherwise volunteer character of services performed by employees of such agencies pursuant to said agreement. For example, where Town A and Town B have entered into a mutual aid agreement related to fire protection, a firefighter employed by Town A who also is a volunteer firefighter for Town B will not have his or her hours of volunteer service for Town B counted as part of his or her hours of employment with Town A. The mere fact that services volunteered to Town B may in some instances involve performance in Town A's geographic jurisdiction does not require that the volunteer's hours are to be counted as hours of employment with Town A.
29 C.F.R. § 553.105