There don't appear to be any commercial SIP clients that support P2P calls over IPv6. (Calls that travel through a central server don't obviously cry out for end-to-end connectivity, though for some reason the ability to connect to them over IPv6 is touted as if it were the be all and end all of SIP over IPv6.) Of the free clients only Linphone and Jitsi claim support, Linphone appears to have had exactly one release (3.5.1-2, available on their server but not linked) with working P2P IPv6, the others fail silently without attempting any outbound connections (and won't connect over IPv4 while IPv6 is enabled), Jitsi throws out a Java error if asked to make a P2P connection. Jitsi does have a bug filed on the subject with priority "urgent" but several other "urgent" bugs have gone untouched for years, and the Jitsi project head made it rather obvious in an interview that he doesn't consider breaking IPv6 support to be a terribly big deal.It's strange considering this is the single most obvious use of an end-to-end connection.