The best way to deal with this kind of problem is to call the SIP provider and ask them if they have a configuration that supports Asterisk based systems. I have yet to come across on that doesn't. They may not have certified the emetrotel system yet nor has emetrotel done the interop testing with them but that doesn't mean that the carrier hasn't made it work for someone else in the past. If the have, they will have notes on the proper configuration and should be more than happy to share the info with you.
If they authenticate using username and password, try starting with the config for voip.ms. That's how they do it and AT&T is probably very similar. If you can't figure it out, call them and ask.