SPA112 is declared to be IPv6 upgradeable. But no such upgrade is available as far as I know. Do you have SPA112 with IPv6 support ?
On the other side, why you are asking for something like it ? SPA112 is not suitable for deployment into public IP addresses. For private, closed, LAN you can use private IPv4 addressing scheme.
In advance, IPv6 is brand new feature for Asterisk 1.8. There are some issues on the Asterisks side. You should consider to avoid IPv6 for production deployment.