Dual-stack is not a compatibility guarantee
A phone may resolve IPv4 and IPv6 destinations while the PBX, carrier or network supports only certain combinations. Identify the address family actually selected for signaling and the media addresses negotiated in SDP.
Inspect separate call legs
A PBX can terminate one leg and originate another with different capabilities. Successful signaling over one family does not prove the media path uses it or can reach the peer. Compare both sides of the intermediary.
Apply policy consistently
IPv4 firewall rules do not automatically govern IPv6 traffic. Review every enabled management, signaling and media listener. The absence of traditional NAT is neither a security guarantee nor a reason to expose all services.
Validate the supported topology
Test registration, calls in both directions, sound and recovery using the intended DNS records. Where interoperability is incomplete, choose a documented supported path rather than leaving partial dual-stack behavior unexplained. Record firmware and PBX version applicability with the result.
Use explicit Asterisk transports for dual stack
Asterisk documents separate IPv4 and IPv6 PJSIP transports for a combined deployment and warns against using one wildcard IPv6 transport as a shortcut. The Asterisk dual-stack guide follows that boundary through signaling and media.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.