Sending an SMS upon entry?

Sounds like you're having an issue with people getting out on time maybe? Sounds pretty overkill to have a message sent every time someone enters a facility. Just wait until you get that tenant who doesn't have unlimited text messages and comes in griping about their phone bill.

If it's an issue with people getting out on time you should be going over gate hours during the lease process and have a sign on/near your gate clearly stating the hours. If they're staying in after hours and not able to get out which is resulting in a manager being bugged constantly enforce a fee. We charged $50 for someone stuck in the gate after hours with my previous company. You could pay the fee or park your car and walk.