Anchor the deep dive to the exact 3CX v20 screens you are likely to touch.

These are the main 3CX interface locations tied to this topic. Use them to shorten navigation time before you move into host, DNS, firewall, or provider-side checks.

• Web Client > switch to Admin Console for system-wide telephony changes, trunks, users, call handling, and integrations.
• Admin Console > Voice & Chat > SIP Trunks > select the trunk > General, Options, Inbound Parameters, or Outbound Parameters.
• Admin Console > Advanced > review Network, E164 Processing, CID Rules, Console Restrictions, or other advanced voice controls.
• 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.

Define the boundaries before production starts moving.

This work succeeds when design choices, authority boundaries, and recovery assumptions are explicit. Good architecture is the control plane that lets operations stay predictable under pressure.

• Inventory every dependency touching Firewall, including hidden dependencies such as service accounts, forwarders, trusts, certificates, or delegated admin workflows.
• Freeze unrelated high-risk change during major milestones so the signal stays clean.
• Write success evidence in operator terms: sign-in works, name resolution is stable, management is reachable, and rollback remains viable.
• Assign clear stop points where the team can pause if Split DNS validation fails.

Baseline the current state before introducing change.

Capture health and inventory first. The baseline is what lets you tell the difference between expected coexistence noise and a real regression.

• Export current configuration, ownership, and topology for Firewall and adjacent services.
• Measure present-day health for Split DNS, logs, alerts, and operator runbooks before starting the move.
• Document exceptions, unsupported integrations, and any legacy dependency that still relies on Remote Access behavior.
• Prepare pilot scope, rollback path, maintenance windows, and evidence collection locations.

Command path:

• sudo systemctl status 3CX* --no-pager
• sudo ss -tulpn | egrep ':(5060|5061|5090|5001|9000|9001)'
• sudo journalctl -u 3CX* --since '-2 hours' --no-pager
• Resolve-DnsName example.com -Server 10.0.0.10
• Get-DnsServerZone
• Get-DhcpServerv4Scope

GUI path: 3CX Admin Console > open the owning area for trunks, users, call handling, storage, backup, reports, or integrations before changing host-side dependencies.

Sequence the change so dependencies remain testable.

Use tight sequencing. Every state change should have an immediate validation step before the next dependency moves.

• Build the target path for Firewall alongside the current path where coexistence is supported.
• Shift one authority plane at a time: service binaries, configuration, identity bindings, name resolution, then client or workload targeting.
• Keep old and new control paths observable until Split DNS proves stable across more than one test path.
• Record every manual change so rollback does not depend on memory during a tense cutover.

Testing has to cover behavior, not just status lights.

Operator-ready testing includes positive flow, negative flow, failure injection, and recovery checks. Green dashboards alone do not count as signoff.

• Test from at least two user or workload paths and one admin path so Firewall is validated from different failure domains.
• Confirm Split DNS behavior under normal load, after a restart, and after a forced rediscovery or cache flush event.
• Run failover, referral, replication, or reconnection tests where Remote Access makes the difference between stable and fragile.
• Document exact commands, UI checkpoints, and evidence artifacts required for final acceptance.

Command path:

• sudo systemctl status 3CX* --no-pager
• sudo ss -tulpn | egrep ':(5060|5061|5090|5001|9000|9001)'
• sudo journalctl -u 3CX* --since '-2 hours' --no-pager
• Resolve-DnsName example.com -Server 10.0.0.10
• Get-DnsServerZone
• Get-DhcpServerv4Scope

GUI path: 3CX Admin Console > open the owning area for trunks, users, call handling, storage, backup, reports, or integrations before changing host-side dependencies.

Plan the move back before you plan the move forward.

A real cutover plan defines what changes, what stays stable, and what triggers immediate stop or rollback. That discipline keeps the team from improvising under pressure.

• Choose the lowest-risk cutover point, then pre-stage DNS, client targeting, monitoring, and communications around it.
• Define rollback triggers in terms of observed user impact, replication health, queue depth, auth failure rate, or management loss.
• Keep the old path read-only, isolated, or otherwise protected from accidental split ownership during coexistence.
• Capture post-cutover evidence before decommissioning anything that would make reversal harder.

Finish by making the new state supportable.

The project is not done when traffic moves. It is done when the operations team can monitor, back up, troubleshoot, and recover the platform with confidence.

• Update monitoring, backup, alerting, and audit coverage for Firewall and all new dependencies.
• Refresh operational runbooks, breakglass access, and access reviews for teams that own Split DNS.
• Retire stale records, abandoned automation, and outdated references that still point at the replaced service path.
• Schedule a post-change review to turn the migration evidence into a reusable operating standard.