Actually I working with MS-MPC for NAT64 stateful on MX960. Well, I have two users only IPv6 and a user only IPv4. I show the results:
1.Video Conference from user only IPv6 - user only IPv6 = bad. It works at times, after several attempts to call.
2. Video Conference from user only IPv4 - user only IPv6 = bad. It works at times, after several attempts to call.
3. Video Conference from user only IPv6 - user only IPv4 = Good!.
Skype is NOT SIP. It runs over Microsoft proprietary protocol MSNP24 and there is no ALG for MSNP. https://en.wikipedia.org/wiki/Microsoft_Notification_Protocol#MSNP24 Which means there is nothing You can do about Skype apart from perhaps ditching NAT64 and introducing 464-XLAT but 464XLAT requires support in UE and on the provider side https://www.juniper.net/documentation/en_US/junos/topics/concept/nat-464xlat.html
Thanks Alex. Actually, I am testing nat64 skype is a problem today for IPv6.