Yes, make that FreeSWITCH or Asterisk box have both IPv4 and IPv6 and the SIP clients will happily route through them. That is that both SIP and RTP ends up on the central dual-stacked box, and the legs between the IPv4-only and IPv6-only client are at the sides of the central box. I've got this working on FreeSWITCH at least, have not peeked at Asterisk for a while due to the lazy lack of IPv6 support and FS has so many other cool and great features.