Names are part of the deployment
Phones and PBXs can use DNS to locate registrars, proxies and supporting services. SIP server discovery can involve records beyond a simple host address, depending on URI, transport and implementation behavior. Confirm what the endpoint actually queries.
Diagnose at the failing client
Compare resolution from the phone's or PBX's configured resolver, not only from an administrator's laptop. Split DNS, VPN routing and cached answers can produce different destinations. Check both address family and selected transport.
Avoid a misleading workaround
Replacing a hostname with a fixed IP can bypass service discovery, failover and TLS identity expectations. Use it only as a bounded diagnostic when appropriate, then restore the supported naming design. A carrier's current address may not be its permanent service contract.
Validate changes over time
Record relevant TTLs and observe recovery after cache expiry or controlled resolver changes. Test incoming and outgoing behavior independently. Keep certificate names aligned with the configured service identity, and include DNS availability in the PBX recovery plan.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.