At the time, you could only use twitter via sms. It's since become much less tied to SMS. I'm not even sure if you CAN do the whole signup and user creation via txt message alone anymore.
The problem with txtmob was the group creation process was always clunky. If you could solve that, make it easy to create groups and start participating, then it might still take off. At one point we even had a version of txtmob which let you sign up and sign off via asterisk, using voip. Looks like pretty much what you did. That was after twitter launched, so we never really put a lot of work in to making it stable.
I still think it's a good idea, i say go for it! Hosted VOIP as a service makes it MUCH easier to do this today, than in 2004-2006.
 - after reading that I do remember hearing about this while reading about the beginnings of Twitter. Unfortunately when brainstorming domains I did not recall TXTMob.