I'm almost certain that servers specify static addressing (or rather, a static IP per service) under IPv6.
In your case, I open the ports to the subnet.
Your only solution would to be statically address the WAN and the VoIP endpoint. If it's a server...you have issues.
As an alternative, you might look into SIP connection tracking.
This thread describes a user's success by turning it off with the value of 0. You would test setting yours to the value of 1.
If it is an inbound service, you may consider a DDNS provider to produce an AAAA record as the subnet updates.