An old PBX is rarely a fire. It is a slow leak: maintenance contracts that creep up, vendors who go quiet, features your team wants and cannot get, and a closet appliance that nobody wants to touch. Replacing it sounds risky because most people imagine ripping everything out on a Friday night. That is not how it works when you do it right. Here is the order we run a legacy-to-VoIP migration for our customers.
The pattern below works whether you are moving off an Avaya IP Office, a Mitel MX-One, an NEC SV9100, a Cisco CallManager Express, or any of the dozens of older systems still in production. The technology specifics differ. The migration sequence does not.
Start with an honest inventory
Before you talk to any provider, write down what you have. Not from memory - walk the building.
- Phone numbers. Main lines, fax lines, alarm lines, elevator lines, every DID. Get them from the carrier bill, not the PBX programming.
- Users and extensions. Who has a phone, who shares one, who needs voicemail, who has a desk and never picks up.
- Call flows. Auto-attendant menus, ring groups, time-of-day routing, after-hours behavior. Take screenshots of the current programming if you can.
- Physical lines. PRIs, analog lines, fax lines. Each one has a contract that needs to be cancelled at the right time.
- The non-phone stuff. Door buzzers, paging speakers, fax machines, modem lines for alarm panels or credit card terminals. These break first if you forget them.
- Integrations. Does the PBX feed a CRM? An EHR? A call recording archive? Map the upstream and downstream systems.
- Conference and overhead paging. Easy to forget. The overhead pager wired into the PBX needs a SIP path or a different solution.
How long the inventory really takes
For a 25-person single-site office, plan a half day. For a multi-site or 100+ user business, plan two to three days, including walking each floor and confirming what is actually plugged in versus what the system thinks is plugged in. We have found alarm lines nobody knew were live, fax machines paying $35/month each that nobody had used in two years, and conference room phones that had been disconnected since the last remodel.
Decide what stays mobile and what stays on a desk
Before you spec the new system, decide who actually needs a desk phone in 2026. For a 40-person office, the answer is usually 15 desk phones plus mobile lines for the rest.
- Desk phones for reception, conference rooms, dedicated phone-based roles (support, dispatch, scheduling). On All-Inclusive Phone Service at $32/user/mo.
- Pro Mobile for anyone whose job is not at a desk: sales, field, executives, technicians. $42 to $62 per user/mo through Pro Mobile.
- Per-Minute Phone Service at $15/user/mo + $0.025/min for occasional users.
The mistake here is buying the same phone for every desk. A bookkeeper does not need a $989 conference phone. A receptionist does need a real handset with a sidecar for transfers. Spec to the role.
Hardware budget for a typical 40-user migration
Realistic example numbers from a recent customer:
- 1 Yealink T54W for reception ($289) + 1 sidecar
- 2 Yealink CP965 for boardroom and main conference ($989 x 2 = $1,978)
- 12 Yealink T46U for support, dispatch, and dedicated desks ($269 x 12 = $3,228)
- 4 Yealink T33G for break rooms and shared desks ($125 x 4 = $500)
- 2 Yealink W73P DECT cordless for warehouse roaming ($185 x 2 = $370)
- 21 Pro Mobile lines at $48/mo (no upfront hardware cost - SIM/eSIM on existing phones)
Total upfront hardware: roughly $6,400 once. Recurring service: roughly $1,800/month across the tier mix. The old PBX was costing $3,200/month between service, maintenance, and per-line PRI fees. Payback on the hardware came in under four months.
Confirm the network is ready
VoIP is sensitive to jitter and packet loss in ways analog and PRI service was not. Before you book a cutover date, run through:
- Bandwidth. 100 Mbps symmetric or better for a 25-seat office. Cable internet works; fiber is better.
- QoS on the router. Voice traffic needs priority. If your router cannot do DSCP-based QoS, replace it before cutover, not after.
- Switch PoE. Most modern IP phones (T46U, T54W) want PoE. If you have a non-PoE switch, budget injectors per phone or upgrade.
- Wiring. Each phone wants a wired ethernet drop. Daisy-chaining through the desk PC's pass-through works but adds failure modes.
- Failover. A single internet circuit is a single point of failure. If you cannot have a backup circuit, configure failover routing to mobile apps and Pro Mobile lines.
- WiFi for the W73P. Cordless phones use DECT, not WiFi, but the base station needs ethernet placement. Plan coverage like a small mesh problem.
Port numbers in the right order
Number porting is the part that scares people. It should not. The sequence:
- Sign with the new provider and stand the system up in parallel. Numbers stay on the old PBX.
- Test the new system with a handful of new (temporary) numbers. Build the auto-attendant. Train the team.
- Submit port orders. We charge $15 per number. Carriers typically take 7 to 14 business days.
- On port date, the numbers flip. Inbound starts hitting the new system. Old PBX goes quiet.
- Cancel old carrier service after numbers are confirmed ported. Not before.
If you cancel the old service before the port lands, you lose the numbers. Do not skip the parallel-run phase to save a month of overlap.
What porting actually looks like under the hood
Number porting is a regulated process. We submit a Letter of Authorization with your most recent carrier bill. The losing carrier confirms ownership and authorizes the port. The new carrier (us) schedules the port for a specific date and time. On port day, the numbers move from one routing table to another. From the outside, calls just start going somewhere new. We monitor the port live and call you immediately if anything looks off. Failed ports happen - usually because the bill name does not match, the account number was wrong, or the losing carrier rejects the request. We have seen all the common rejections and know how to fix them.
What we run for customers
A typical migration for a 25 to 50 seat office:
- Week 1: Discovery call, inventory, quote. Confirm internet capacity. A reliable 25-seat office needs at least a 100 Mbps connection with low jitter.
- Week 2: Hardware ordered. PBX programmed in our portal. Call flows mirrored from the old system.
- Week 3: Phones land pre-provisioned. Install on desks. Test with temporary DIDs. Train staff on apps and voicemail.
- Week 4: Port orders submitted. Active monitoring of the cutover.
- Week 5: Port lands. Old PBX powered down after 48 hours of clean inbound on the new system. Cancel old carrier.
The whole thing is 4 to 6 weeks for most offices, and zero of that is unplanned downtime.
Industry-specific gotchas
The general migration playbook is the same. The risks change by industry.
- Property management. Maintenance request lines, after-hours emergency routing, per-property number tracking. Plan integrations with AppFolio, Buildium, or Rent Manager before cutover. Property management workflows documented separately.
- Healthcare and dental. HIPAA-compliant routing, BAA on file, recording rules per state. The dental practice and healthcare pages cover what changes.
- Legal firms. Per-attorney DIDs, intake routing, calendar-aware routing for court hours. Clio integration details on the integrations pages.
- Field service. Dispatch routing, on-call rotations, customer callbacks from technician mobiles. Pro Mobile is usually the right answer for the field side.
- Multi-location. Extensions dial across sites. Local DIDs per location. After-hours routes between locations on a single hunt group.
- Real estate. Agent forwarding, listing-specific numbers, marketing call tracking. See the real estate page for workflow details.
What breaks if you skip steps
- Forgotten analog lines. Door buzzer goes dead because nobody mapped it. Fix: budget for ATA adapters or POTS replacement.
- E911 misconfigured. Address on file does not match the desk. The misdial fee is $150 if a phone calls 911 and there is no valid registered address - federal carrier passthrough, not a markup.
- Internet wasn't ready. Audio dropouts on day one. Fix in advance: QoS on the router, a wired connection to every desk phone if possible.
- Training got skipped. Staff makes the new system look broken because they cannot figure out voicemail. Fix: an hour of live training per location.
- Old PBX kept ringing. If the old system still has live numbers pointed at it, customers ring a dead box. Track the port confirmation per number.
- Voicemail messages lost. Old PBX voicemails are usually not portable. Tell the team to clear inboxes before cutover or warn them messages will stay on the old system until decommissioned.
- Fax stops working. Analog fax over VoIP is unreliable. Plan to move to vFAX before cutover, not after the fax machine dies.
- Speed-dial lists wiped. Users had personal speed dials programmed on the old phones. Document them or expect complaints.
What to ask a provider before signing
- Are you an operator or a reseller? Resellers add hands-off layers. We run our own platform out of Ocoee, FL.
- Who programs the call flow? If the answer is "you do, in the portal," that is fine if you have someone who will. If you want the provider to mirror your old system, confirm that is included.
- What is the contract length? Month-to-month with no termination penalty is the right answer. Three-year contracts with penalties exist - read the fine print.
- Will you sit on the cutover? A live engineer on the port date catches problems immediately. We do this for our customers.
- What is the support response time after cutover? The first 48 hours after porting are when issues surface. Make sure support is reachable.
- Are integrations included or extra? Microsoft Teams Direct Routing, CRM integrations, vertical apps - confirm scope.
- What's the SLA on uptime? Numbers vary. Make sure you understand how it is measured.
- Do you have a project plan template? If they shrug at this, you are the project manager. Decide whether that is okay.
Common mistakes customers make on their own
- Picking a cutover date around a quarter-end or month-end. Avoid your busiest week. Pick a Tuesday in a slow stretch.
- Not telling staff the cutover is happening. Surprise migrations look like outages. Email the team a week ahead with the date and a one-page user guide.
- Trying to rebuild the call flow from memory. Document the old flow before turning it off. Mirror it. Improve it after the new system is live.
- Cancelling the old carrier before the port is final. The worst mistake. The numbers belong to whoever the carrier says owns them. Until the port lands, the old carrier is the only path.
- Buying for last year's headcount. If hiring is in motion, size for 12 months forward.
- Forgetting executive assistants. EAs need ring groups, shared-line appearances, and call screening. Skipping them means the CEO calls IT angry on day one.
- Skipping a pilot. 5 to 10 pilot users for a week before the big cutover catches policy and provisioning issues at low cost.
The day-of-cutover playbook
What we actually do on port day:
- 6 AM ET: Engineer online. Old system still active.
- 7-9 AM ET: Port window. Carrier executes the port. Inbound traffic begins flowing to our platform.
- 9 AM ET: Test calls from outside lines confirm inbound is hitting the right destinations.
- 9-10 AM ET: On-site contact (or remote contact) does a one-hour live walk-through with reception and key users.
- Throughout the day: Engineer monitors for issues. Most days are quiet.
- End of day: Status report to the customer. Old PBX stays powered up for 48 hours as a fallback.
- 48 hours later: If no rollback events, old PBX gets powered down. Cancel old carrier service.
Where to start
Send us your current carrier bill and a rough count of users by role. We will quote the new platform side by side with what you pay today and tell you what porting your specific numbers looks like. Compare options on pricing or hit get started to schedule discovery. If you want a peer comparison first, how we compare to Nextiva and how we compare to RingCentral cover the usual alternatives.