We are keen to test IPv6 VoIP services, but initial testing has found a slight flaw in the logic!
Our SIP server handles IPv6, but it passes RTP between enpoints to negotiate for themselves. That works fine for codecs as we say customers have to support alaw and all carriers support alaw, so unless other preferences are agreed it is alaw.
However RTP pass-through does not work well for IPv6. In theory each end is told what to send to the other. And if one end says "send me IPv6" the other should. The problem is that the other end does not understand IPv6 and so the call fails. Sadly our call carriers are not IPv6 yet, so even if we support IPv6 and our customers support IPv6, it won't work.
To solve this we are working on a new generation of call server. It will have a simple back to back media gateway to allow IPv4/6 mapping. This may be a media forwarding box, or a special case of our firewall/routers creating customer mapping sessions. We have not decided yet.
However, we just want to re-assure customers we are commited to full IPv6 support for VoIP services.