SMS is likely to be the source of communications with SIP accounts, and basic phones. This is very basic, and uses the cellular voice network to send the SMS messages. With that said, most carriers do not charge any charges for receiving or sending SMS messages. If you have most smartphones, you will likely send an RCS message automatically. RCS is more versatile, but uses data for communications. Also, RCS is free everywhere that the carrier and device can handle it. My SMS number is +1 4126829751.