Sure this has been talked about before, but I couldn't find it (the forum searches has always failed me).
Feature: sms-activated callback
Think a really simple protocol, like
user
ass theirnumber[@provider] [mynumber[@provider] [pause]]
mynumber would be optional and default to the CID.
user
ass could be a really simple token or even omitted
if combined with a well-known cid (if I, say, preregistered
my cellphone)