Well, I can't vouch for the 770 part. But I have installed it on my N800 and have it connecting to an asterisk server running 1.4.4. It has some issues registering, and the voice quality is ok. But all in all its working. So I'm guessing as they work out the bugs its only going to get better!

Under the advanced options I turned the registering from 'auto' to 'options' and that seemed to work. I've been able to make both outgoing calls and receive incoming calls