A method and system for preventing replay-type attacks on a vehicle communications system that sends short message service (SMS) messages between a call center and a fleet of vehicles. The method uses separate sequence counters maintained at the call center and at each of the vehicles in the fleet to help prevent or at least minimize the effects of unauthorized third party interference; such as replay-type attacks. Each wireless message is embedded with a sequence counter that is provided by the sender and is compared by the recipient with a separate sequence counter for purposes of validation. Some optional features that can be used in conjunction with the sequence counters include a tolerance window feature, a consecutive message feature, and a proximity feature, to name but a few.