SMS is essentially just email. The phone number just ends up being part of the email address: 0123456789@att.net and whatnot. Edit: there was also a time when

SMS is essentially just email. The phone number just ends up being part of the email address: 0123456789@att.net and whatnot. Edit: there was also a time when AIM could send and receive SMS due to this structure, and at one point it was trivial to set up a script to email your phone number as SMS for alerts and such. This became more complicated as SMS providers started to require a lot of the same stuff as email providers to cut down on spam.