The over the air part of SMS can be zero cost. The SMS message is in a spare 128byte segment of a standard phone-base status message (it can send a new packet s

The over the air part of SMS can be zero cost. The SMS message is in a spare 128byte segment of a standard phone-base status message (it can send a new packet specificaly to transmit the SMS but it doesn't have to if you don't mind a few mins latency). If the receiver is on the same base station -like kids texting each other across the classroom it doesn't use any of the phone companies network bandwidth. It takes more bandwidth to send a message back to HQ to bill for the text message than it does to transmit the SMS.