I recall reading long ago that SMS messages were a hack built on top of the unencrypted "paging" channel in the protocol. They're received and "ignored" by all

I recall reading long ago that SMS messages were a hack built on top of the unencrypted "paging" channel in the protocol. They're received and "ignored" by all phones near the intended recipient. IIRC, the paging channel is a broadcast channel that was originally used to send short message to a handset to advise them of an incoming call and to request a private channel to receive it. It had lots of excess capacity so someone had the idea of pushing short text messages over it.