"IPv6 Support in Cisco Unified Communications Devices"
You can enable IPv6 on your phones by going to CCAdmin, Device > Device Settings > Common Device Configuration
IP Addressing Mode Preference for Signaling: IPv4 (default)
Apply the common device configuration on the phone, save it and reset it.
I try it's not working.
My all 9971 Phone register with CUCM 9.1 in IPv4.
I can not find the option in the network configuration on the phone.
Arun Prakash Sharma