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Field guide · Voice & telecom

POTS to VoIP: What Breaks

Carriers are retiring copper and the quotes make it look like a phone swap. It is not. The desk phones are the easy part — what catches people out is everything else that was quietly sharing those lines: the elevator, the fire panel, the alarm dialer, the fax that a regulator still expects to work. This is the survey to do before the cutover date, and what each of those lines actually needs instead.

Diagram

The lines nobody remembers until they stop working.

01Moves cleanly

Replaced by the new system with no separate decision needed.

  • Desk extensionsThe part that is genuinely a phone project
  • Reception and hunt groupsRebuilt as ring groups, usually better than what they replace
  • VoicemailMoves, and stops being a machine in a cupboard
  • Conference room setsCheck PoE budget on the switch before ordering
02Needs a different answer

Works over analog and does not simply transfer. Each one is its own small project.

  • FaxWorks until it does not. T.38 or keep one analog line — decide, do not hope
  • Card readers and payment terminalsMany dial out. Confirm the processor supports IP before the line goes
  • Overhead pagingUsually an analog port on the old system. Needs a gateway, not a handset
  • Modems on plant equipmentOften the only path a vendor supports for remote service
03Does not move

Regulated, monitored, or required to work when the power does not. Plan around these.

  • Elevator emergency phoneCode-required, must work in a power cut. Treat as immovable
  • Fire alarm communicatorMonitored line with its own inspection regime. The AHJ decides, not the carrier
  • Gate and barrier phonesLife-safety adjacent and frequently forgotten until access fails
  • Pump and sprinkler supervisionSame category as the alarm. Do not cut without the monitoring company

Inventory from the carrier bill, not from a walk-round counting handsets. Every line in the third column bills exactly like a desk extension and has nothing on the wall to find, which is why they are discovered during the cut rather than before it.

Fig. 01What moves cleanly, what needs its own answer, and what does not move at all.

Procedure

Doing it

The survey, in the order that keeps the life-safety items from being discovered last. Each step is a thing that has its own owner, its own lead time, and its own way of going wrong.

Tools
  • Butt set or lineman's handset
  • Toner and probe
  • Tone generator for line identification

  1. Inventory every line, not every phone

    Get the carrier's circuit list and walk the building against it. The count almost never matches. Lines that appear on a bill and nowhere on a desk are the ones that matter — they are feeding equipment, and equipment does not complain until the day it needs to dial out.

    CautionA line with no dial tone today may still be a monitored circuit. Do not mark it dead until you know what is on the far end.

  2. Find the elevator line and treat it as immovable

    Elevator emergency phones are required to reach a monitored answering point, and in most jurisdictions the line has to survive a building power failure. A standard VoIP ATA on a desk UPS does not satisfy that. This is a conversation with the elevator maintenance company and the AHJ before it is a telecom decision.

    CautionCutting an elevator line without a compliant replacement can take the car out of service. Confirm the replacement is accepted before the copper is disconnected.

  3. Identify the fire alarm communicator

    Older panels dial out over two POTS lines. Modern practice replaces them with a cellular or IP communicator listed for fire use — not with a generic ATA. The panel's monitoring company has to be involved, the change usually needs to be recorded, and in many places it needs an inspection afterwards.

    CautionA fire panel that cannot report is a life-safety fault. Treat it with the urgency that implies. Never cut this line to see what happens.

  4. Chase the alarm, gate, and pump lines

    Burglar alarm dialers, gate intercoms, sprinkler tamper and flow switches, sump and generator autodialers, and building automation modems all sit on copper in older stock. Each one either gets a listed IP or cellular path or stays on a retained line.

  5. Decide what happens to fax and card readers

    Fax over VoIP works or does not depending on the codec and the far end. If the fax is genuinely required — some medical and legal workflows still are — either keep one analogue line or move to a fax service with a real number. Old card terminals dialing out are the same problem and usually the cheaper fix is a network terminal.

  6. Check power, then test before you cut

    Copper carried its own power. Everything replacing it needs an outlet, and the switch feeding a PoE phone needs to be on a UPS or the phones die with the lights. Stage the SIP service alongside the copper, port a test number, and place real calls — including to the alarm receiver and the elevator answering point — before the disconnect date.

    CautionBook the copper disconnect only once the cutover is proven. Doing both on the same day leaves you nothing to fall back to. A carrier disconnect is far harder to reverse than a delay.

Questions

Questions people actually ask

Can I just put an ATA on the elevator phone line?
Usually not, and it is not a telecom judgement to make alone. Elevator emergency communication generally has to work during a building power failure and reach a monitored answering point. A generic ATA plugged into a desk outlet fails both tests. Involve the elevator maintenance company and the authority having jurisdiction, and get the replacement accepted before the copper goes.
What about the fire alarm panel?
It needs a communicator listed for fire alarm use — cellular or IP — installed and registered with the monitoring company, not a consumer ATA. Depending on where you are the change may need to be recorded with the AHJ and re-inspected. Treat the panel as the first item on the survey, not the last.
Does fax work over VoIP?
Sometimes, and unpredictably. T.38 helps, G.711 pass-through sometimes works, and both depend on the far end and the network in between. If the fax genuinely has to work, keep one analogue line or move to a fax service. Do not discover this on the day.
How do I find what a mystery line is connected to?
Tone it. Put a tone on the pair at the block and trace it with a probe — that is what tells you it terminates at the sprinkler riser rather than on somebody's desk. Our toning and tracing guide covers the method, including how to work a live pair safely.
Will the phones work in a power cut?
Only if you plan for it. A copper line drew power from the exchange, which is why the phone in the closet still worked when the building was dark. IP phones need the switch, the router, and the internet circuit all to be on battery. Size the UPS for the whole path, not just the handset.

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