Split DNS for the 3CX FQDN works externally and fails for internal phones after v20 readiness changes.
Use this when remote users connect fine and on-prem phones or admins hit the wrong address because internal resolution never changed with the platform.
Start from the failing condition, not the loudest symptom.
Use this when remote users connect fine and on-prem phones or admins hit the wrong address because internal resolution never changed with the platform. Treat the visible error as the end of the chain and work backward until the first dependency that actually moved is obvious.
What To Verify
Separate healthy dependencies from the one that actually broke.
Capture one failing path, one known-good path, and the exact change window before touching configuration.
Confirm Split DNS state from both the control plane and an affected workload so you are not troubleshooting a cached or partial view.
Confirm FQDN state from both the control plane and an affected workload so you are not troubleshooting a cached or partial view.
Confirm DNS state from both the control plane and an affected workload so you are not troubleshooting a cached or partial view.
Record which dependency actually moved first: identity, name resolution, transport, policy, or runtime state.
Working Sequence
Recover the path without widening the blast radius.
Prove the break is centered in Split DNS before editing the next layer down the dependency chain.
Correct the narrowest failing state first, then retest from the same path that originally failed.
Re-register, reload, or restart only the component tied to FQDN rather than stacking broad changes together.
Validate with a second client, site, or node so the fix is not limited to one warm cache or one host.
Capture the final health evidence and the triggering condition so the next incident starts from facts instead of memory.
GUI Locations
Go straight to the 3CX screens that actually own this change.
Use these paths as the fastest starting points in the 3CX v20 interface before you widen the search into network or host layers.
Web Client > switch to Admin Console for system-wide telephony changes, trunks, users, call handling, and integrations.
Admin Console > Advanced > review Network, E164 Processing, CID Rules, Console Restrictions, or other advanced voice controls.
Admin Console > Users > select the user > General, IP Phone, BLFs, Rights, Voicemail, View, or Options.
Admin Console > Users > select the user > IP Phone to provision, reassign, or review phone settings.
Outside 3CX: update internal DNS, public DNS, and the firewall or SD-WAN console for split-DNS, NAT, SIP ALG, and port-forward changes.