Mont. Code § 19-2-603
Except as otherwise provided in chapter 3, part 21, of this title and this section, a person who again becomes an active member of a defined benefit plan subsequent to the refund of the person's accumulated contributions after a termination of previous membership is considered a new member without previous membership service or service credit. The person, while either an active or inactive vested member, may reinstate that membership service or service credit by redepositing the sum of the accumulated contributions that were refunded to the person at the last termination of the person's membership plus the interest that would have been credited to the person's accumulated contributions had the refund not taken place. If the person makes this redeposit, the membership service and service credit previously canceled must be reinstated.
§ 19-2-603, MCA