Answer
Migration runs in six phases: audit the existing carrier contract and PBX inventory, validate the network for VoIP, select a UCaaS provider, port numbers, run a parallel pilot, then schedule the production cutover [1][2]. End-to-end timelines run 6 to 16 weeks for mid-market deployments [3]. The single highest-risk step is number porting — most projects schedule it as the final cutover task with the legacy PBX still live as fallback [4].
How to
Get the contract end date, auto-renewal window (often 30–90 days out), and early-termination fees from your current carrier before doing anything else. Knowing this opens negotiation room with the new provider.
Document every active phone number per location, distinguish voice from fax/elevator/alarm lines, and capture user preferences: desk phone, softphone, or mobile-only. This sizes the license and hardware order accurately.
Confirm bandwidth (~30 Kbps per concurrent call), turn on QoS to prioritize voice, upgrade switches to PoE if you'll deploy desk phones, and verify the firewall passes SIP traffic without ALG mangling.
Match the shortlist against required features (call recording, AI, integrations, compliance). Map every IVR menu, hunt group, and auto-attendant from the legacy PBX into the new platform's admin portal.
Open Letters of Authorization to port numbers — porting takes 2 to 6 weeks per carrier. While porting clears, run 10–20 pilot users on the new platform alongside the legacy PBX to surface issues.
Schedule the production cutover for an off-hours window, validate inbound and outbound calls from every site, keep the legacy PBX powered for 7–14 days as fallback, then decommission hardware and cancel carrier services.
Tools and prerequisites
Source of call flows, extensions, and number assignments to recreate in UCaaS
Required to validate ownership and submit port orders
Measures bandwidth, jitter, and packet loss before cutover
Power IP desk phones over the data cable without separate adapters
Where you build users, call routing, and auto-attendants in the new platform
Tracks port dates, pilot users, cutover steps, and rollback triggers
Common mistakes
Key terms
Workstream
Process of transferring existing phone numbers from the legacy carrier to the new UCaaS provider while keeping numbers live.
Workstream
Test phase where a subset of users run on UCaaS while the on-prem PBX still handles production traffic.
Technical dependency
Network configuration that prioritizes voice packets over other traffic to keep calls clean during congestion.
Technical dependency
Switch capability that powers IP desk phones over the same cable that carries data.
Workstream
Scheduled event when production traffic moves from the legacy PBX to UCaaS, usually outside business hours.
Citations
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