Look at packet8. I'd suggest vonage, but they're having MASSIVE capacity issues. Also, keep in mind, e911 sucks. Dont rely on an IP phone to save your life.
If you really wanna get funky, get a linsux box, run asterisk.org PBX and get a local telco to terminate you DID's
ATT generally sucks, but cant comment on their voip service. I'd not wanna be the poor ****er to try. As for bandwidth? The quick/easy answer, is 'You need what your phone modem could transfer'
56k modems are 64kbit channels (up/down) - You need roughly 6kB/s up/down for a lossless phone call. Add compression, iots actually lower, but 64kbit is a good rule of thumb.
In reality? we can use an IP t1 to carry ~35 voice calls. A traditional analog t1 only carries 23 voice calls. divide 1.54mb/s by 35
