The STUN requests and responses needed to get the Google Voice server to send RTP ended up being very basic, essentially just reflecting the STUN username attribute back is all that was required.
I’ve now been able to get RTP audio flowing.
The gotcha with this is that if sipsorcery is ever able to incorporate the XMPP approach to setting up Google Voice calls it’s only going to work for SIP user agents that support ICE. The version of counterpath Bria softphone I use does support a “version” of ICE but I’ve yet to get it to get the RTP flowing. I’ve got some more messing around to do with the SDP that I’m supplying to the bria and hopefully there’s a way to get it to work.
I’m off to do manual labour in the country for the rest of this weekend so fingers crossed early next week I’ll successfully get a SIP call from a Bria softphone receiving audio from an XMPP initiated Google Voice call.