AT&T has wifi only and low data plans on GoPhone prepaid for Smartphones.
The words "wifi" and "ethernet" do not appear on that page.
If you're doing wifi-only, get yourself a static IP, run asterisk, use any old cellphone with SIP support and wifi and skip AT&T, as they are fuckers of the highest degree. Their prices are beyond fucking ridiculous. They want $50/mo for a land line. I got SIP for about ten bucks a month. My Xperia Play is now our cordless phone, and it's also a neato clock. My server is a $20 pogoplug, but in fairness I bought two of them so I could do HA.