After upgrading to Ubuntu 9.10 which includes Asterisk 1.6 my Google Voice connection through Gizmo5 stopped working. Sound familiar?

The issue is that Asterisk 1.6 requires insecure=invite in the sip.conf definition for Gizmo.

Here’s my working config:
[gizmo]
type=peer
context=your-context-here
disallow=all
allow=ulaw
allow=gsm
dtmfmode=rfc2833
host=proxy01.sipphone.com
insecure=invite    <---Change this setting
secret=password-here
username=gizmo-number-here
canreinvite=no

Source: http://vaug.ca/pipermail/vaug/2009-March/000053.html