Hi Terry,
Like I mentioned to you on the phone last week, the carriers do not like VoIP and tend to block it.
If you are using SIP port 5060, you should try something else, otherwise they may have some
other way to detect this on a different port, as they do here in Germany.
I gave-up trying to use 3G. Even with a solid and fast connection, I experienced the same issue
you do. I use a 3G router with an ethernet jack.
73,
Mitch DJ0QN / K7DX