I feel your pain. I've been working with asterisk end solutions and would give anything for a SIP client on the 770. If you run the regular version of Gizmo you can use a second provider and there are details for using it with a SIP provider. But there seems to be no info for the 770

After much googling I found a few mentions of SIP on the 770, such as Tapioca, etc. But it doesn't appear to be anyone actively working on it. Go figure.