Mont. Code Ann. § 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 interest at the actuarial assumed rate of return in effect at the time of the redeposit. If the person makes this redeposit, the membership service and service credit previously canceled must be reinstated.