…Or not so happy.
After spending WAY too long trying to get my sip phones to register on a remote Trixbox, I figured out that the problem was due to my local Netopia router/modem from ATT messing with my SIP traffic. Here is the solution:

Telnet to your Netopia and:

The setting is not in the web gui. You have to telnet into the
router, then type. Each line is a new command.

configure
set ip sip-passthrough off
save
exit
Restart

On the Netopia 4622xlt
Telnet into the device
Use the arrow keys to highlight ‘quick menu’ and press “ctrl-n”
That brings you to the console.
type:

ip nat alg sip enable no
save
exit
restart

That info came courtesy of the following website: http://www.junctionnetworks.com/knowledgebase/onsip/devices/router-configuration/netopia