Hopefully something here will be useful

Configuring Asterisk to connect to Gizmo including Call Out functionality

For initial Asterisk-Gizmo configuration, go here.

That documentation will get you far enough to use Asterisk to make calls to other Gizmo users. What I wanted to do, however, was use Gizmo's "Call Out" feature to make calls through Asterisk to the PSTN. I was unable to find any documentation on setting up Asterisk to make PSTN calls through Gizmo, so I decided to figure it out myself. With the help of Ethereal and Gizmo's client I was able to determine the format of the SIP request Gizmo's servers were expecting. One of the key parts I was missing which caused problems authenticating with Gizmo was the "fromdomain". Apparently they're expecting the From domain in the SIP request to be proxy01.sipphone.com. By default it was using the IP address of my Asterisk server.

Here's a snippet from my sip.conf showing how I configured Asterisk. Make sure to substitute your Gizmo phone number and password where applicable:
[to_gizmo]
type=friend
host=proxy01.sipphone.com
username=17470000000
fromuser=17470000000
fromdomain=proxy01.sipphone.com
secret=YOURPASSWORD
canreinvite=no
reinvite=no
nat=yes


Thank you, Gizmo, for offering a product based on open standards. The ability for me to use Asterisk to connect to your network is the reason I buy Call Out minutes from you.

---