Hi.Sorry for bringing up such an old topic, but I figured I would help you out. I have made a lot of progress since my original post. With the right settings, CSipSimple is awesome, and actually sounds better than some hardware VoIP phones. If you decide to roll your own Asterisk server, I suggest you try using TCP transport for SIP signaling, the G729 codec for mobile data, and the G722 codec for WiFi. There is a huge difference between in battery consumption by using TCP over UDP, especially over WiFi. There is a ton of information here, and it covers how you can set up an Asterisk server with Google Voice, TCP transport, and the G729 codec. I've been using this since the end of September, and it's great.
This is exactly what I was looking for, but it seems that the website in the link does not work. Do you have another link or can you help me with the configuration of SIP TCP and Google Voice to Asterisk?