''[ SMS'' is a standardized communication protocol that enables devices to exchange short text messages and was defined as part of the 1985 GSM protocol. Originally it was designed to "fit in between" existing signalling protocols which is why SMS length is limited to 160 seven-bit characters. It was subsequently codified into the SMPP signalling protocol that transmits SMS and is limited to precisely 140 bytes (or 1120 bits). ]
You need less that a second of stable signal to send one SMS, but one call needs a stable connection to start a session.