If IPv6 address is used, I-CSCF must be defined in DNS and will be used later in “Record-Route” header of SIP messages. I-CSCF will ask HSS for user information, such as the IP address of S-CSCF. But if IPv4 address is used, I-CSCF will be needed only some scenarios like Roaming scenario, but not for every calls.
For IPv6 enablers, when mobile terminal launches a IPv6 connection to IMS platform, first of all, GGSN(or PDN-GW), a network element in 2G/3G(or LTE) core network, will support IPv6 enabled APN. But for IPv4 traffic, a default IPv4 APN is needed. DNS server will also be different in the IMS core of IPv4 and IPv6. AAAA record is added by 3GPP and RFC standard organization to support IPv6 queries in IPv6 IMS. Since IPv6 address is global routable, it will make the routing mechanism change from client-server (IPv4) towards peer-to-peer (IPv6).
For more information, see: