POTS Replacement: How to Move Off Copper Lines Without Replacing Every Phone

Most organisations that say they want to "get rid of POTS" don't actually want to get rid of their phones. They want to stop paying for copper lines they can't use, and they want the fax machine in the back office, the lift intercom, the alarm panel and the old conference room handset to keep working. Those two things are not the same project, and confusing them is where most replacement budgets get wasted.

Definition: POTS (Plain Old Telephone Service) replacement is the process of retiring analogue PSTN lines while keeping the analogue endpoints — desk phones, fax, intercoms, alarm panels, lift phones, card readers — that depend on them. It is not the same as an IP phone migration.

What POTS lines actually are, and why they are disappearing

A POTS line is a copper pair from the local exchange carrying a single analogue voice channel, powered by the exchange rather than by your equipment. You can plug a fax machine, a modem, a lift intercom, an alarm panel or a simple desk phone straight into it. There is no encryption, no address to configure, and no IP to plan.

That simplicity is exactly why these endpoints are still in service while everything around them has moved to IP. The old alarm panel expects a conventional analogue line feed and a ring voltage. Nobody is going to replace it for the sake of a VoIP project, and it does not care what your PBX architecture looks like.

Meanwhile the telco is retiring the copper. Depending on your market, the line rental may be rising, the copper pair may be moving to a wholesale-only provider, or the line may be converted to an emulated service with a shorter contract. None of that is interesting on its own. What matters for planning is the practical consequence: the analogue endpoint does not care, but its replacement does, and you have to decide which one you are buying.

Three routes off copper, and what each one actually needs

There are three fundamentally different ways to keep an analogue device alive, and each one has a different failure mode and a different cost structure.

1. Use the FXS ports built into the IP-PBX. Several Newrock OM-series platforms ship with analogue FXS/FXO ports on the chassis. The OM20G carries up to 4 built-in FXS/FXO ports, the OM50G up to 12, the OM80E up to 48 and the OM200G up to 96. If your endpoint count fits inside that, you need no separate gateway at all — the analogue phone plugs into the PBX exactly the way it used to plug into the wall socket, and it becomes an extension on the new system. This is the cleanest option when it fits, and the most wasteful when it doesn't, because you are buying analogue capacity on a platform that also does IP.
2. Deploy a dedicated FXS/FXO voice gateway. A gateway does one job: it turns analogue ports into SIP endpoints. The MX8G provides 8 FXS/FXO ports with one WAN and one PC port, both 10/100/1000Mbps. The MX60E-G scales to 48 FXS/FXO ports and adds SIP registration failover, load balancing and PSTN failover, with optional 1+1 redundancy on the network port and power supply. The MX120G reaches up to 96 FXS and FXO ports for multi-tenant and large call centre deployments. The MX60E-G-SC is the IMS-aware variant, with 48 FXS ports, 48 SIP/IMS registration accounts, 32 concurrent calls on G.711 and 16 on G.729A with G.711, and compatibility with mainstream operator IMS platforms. This is the right answer when the analogue population is large, spread across sites, or needs to survive independently.
3. Replace the endpoint with an IP device and keep the function. Sometimes the fax machine is replaced by a fax-over-IP service, the alarm panel by a SIP-capable panel, the lift intercom by an IP intercom. This removes the copper dependency entirely and is the right long-term answer, but it changes the device, the installer, the maintenance contract and possibly the certification of the alarm panel. Budget it as a separate project, not as a line in the phone system quote.

How to size the analogue requirement properly

The models below cover most branch and campus analogue populations. Every figure is taken from Newrock's published product pages.

Most sizing mistakes come from counting "phones" instead of counting port types. Walk the site and record, for every analogue endpoint, which side of the exchange it sits on:

• FXS (analogue phone side): a desk phone, a fax machine, a wall phone, a lift intercom, a card reader, a PBX-to-PBX tie line. The gateway presents a dial tone and battery feed to the device.
• FXO (exchange side): a line from the local telco that you want to keep. The gateway terminates the PSTN line and presents it to the IP side as a SIP trunk or registered account.

Once you have that split, add a margin for the endpoints you will forget. Reception phones, the backup fax, the temporary contractor phone, the second line in the meeting room — they always exist. A common planning rule of thumb is to size gateway ports above the counted requirement, but the number that actually matters is the concurrency figure, and that is a different calculation.

For a gateway that also terminates IMS or SIP, concurrency becomes the binding constraint. The MX60E-G-SC, for example, supports 32 concurrent calls on G.711 and 16 on G.729A with G.711. If your analogue estate is mostly fax and alarm traffic, concurrency will be low and ports will be the limit. If it is a call centre floor of analogue headsets, concurrency is the limit and you need to size on it.

Where a branch needs to survive a WAN outage

This is the question most often missed, and it is the one that determines whether a POTS replacement causes a business interruption or a nuisance.

Consider a branch with a cluster of analogue extensions and a small PSTN line, connected back to a head office PBX over a wide area link. If that link drops, a centralised IP-PBX has two possible behaviours: it either stops serving the branch's phones, or it keeps serving them locally. In the first case, a single fibre cut turns every room in that branch into a dead desk. In the second, the branch keeps making and receiving calls on its own local PSTN and re-registers when the link returns.

The Newrock MA50-LS local survivability branch exchange is built for exactly this case. It provides 2 to 8 FXO ports, registers 50 to 200 SIP users, keeps calls working locally when the head office or cloud server becomes unreachable, and switches back to the central server once the connection is restored. Combined with an MX-series gateway or the built-in FXS/FXO ports of the OM series, that gives a branch a defined failover path rather than an assumed one.

The point to take into your design review: draw the failover behaviour on paper before you order anything. For each site, write down what still works if the WAN fails, what still works if the PSTN fails, and who is responsible for noticing.

Cost components people forget to budget for

The gateway hardware is usually the smallest line in this project. These are the ones that surprise people:

• The analogue cabling. Existing copper runs may terminate on a distribution frame that has no spare ports. New Rock's PT2400 patch panel provides 24 RJ11 ports, 6 RJ45 for equipment and 24 RJ11 for the CO or phone lines, with built-in surge protection; the PT4800 does the same at 48 ports with 12 RJ45. You will need somewhere to land the new field wiring.
• Line rental during the transition. Copper and SIP trunks typically run in parallel for a cutover period. That period is a budget line, not an oversight.
• Number porting and trunk provisioning. Moved numbers, new SIP trunk orders, and carrier lead times all sit on the critical path.
• Endpoint replacement. Any fax machine, panel or intercom you decided to retire rather than convert carries its own cost and its own project.
• Ongoing management. A handful of gateways scattered across branches is a fleet. UMS is used to manage and remotely maintain OM-series IP-PBX systems and MX-series VoIP gateways together, which is what keeps that fleet from becoming an unmonitored liability.

What you will not find in any of these lines is a licence fee for the hardware itself on the OM series, because the platform licenses by device rather than by person. That is a genuine structural difference from per-user licensing models, and it is one of the reasons hardware-based POTS replacement proposals often come out lower in total cost than the software-led alternative.

Newrock perspective

Newrock approaches POTS replacement as a port-mapping exercise rather than a refresh project. The practical sequence is: count FXS and FXO separately by site, decide which endpoints get converted and which stay analogue, then choose between built-in analogue ports on the OM series and a dedicated MX-series gateway based on whether the analogue population is large enough to justify a separate device and how much independence that site needs when the WAN drops.

For most estates the answer is a hybrid. Use the built-in FXS/FXO on the OM series for a small number of legacy devices in the main building, put an MX-series gateway where the analogue population is concentrated, and add the MA50-LS at branches where local survivability is a requirement rather than a nice-to-have. The MX60E-G's SIP registration failover, load balancing and PSTN failover, with optional 1+1 redundancy on both network port and power supply, are what make a gateway deployment defensible in a building where nobody can easily reach the rack.

MX60E-G VoIP gateway and the MA50-LS local survivability branch exchange are the two pages to read first if you are sizing this properly.

FAQ

Can we keep our existing analogue phones?

Yes. That is the point of FXS. The phone plugs into a gateway or into the PBX's built-in analogue port and behaves like an extension. It will not give you the features of an IP phone — no soft keys, no video, no PC screen sync — but it will make and take calls.

Do we have to replace the fax machine?

Not necessarily. Some gateways support fax pass-through and there are dedicated fax-over-IP alternatives. Test the specific machine and the specific line, because fax behaviour varies more than people expect.

How long does a POTS replacement take?

The physical work is usually days. The schedule is dominated by carrier provisioning, number porting and how many sites have to be done without disruption. In a multi-site estate, plan on a phased rollout by building rather than by department.

What happens to our existing alarm panel?

Most panels work on an analogue line and can be moved to an FXS port unchanged. Check with the panel manufacturer first, because some panels require a specific line type, polarity or line state. Treat this as a required conversation, not a detail.

Is FXO or FXS better for keeping an old PBX alive?

FXO if you are terminating a PSTN line from the telco. FXS if you are presenting a line to an analogue device such as a phone, fax or intercom. Getting these two the wrong way round is a common and immediately visible error.

Can the system survive if our internet connection drops?

It depends on the architecture you choose. A branch with a local survivability exchange such as the MA50-LS keeps serving calls when the head office server is unreachable, and switches back automatically once the connection returns. A branch with no local component stops when the link drops. This should be a deliberate decision per site.

Do we need a separate box for every building?

No. The MX60E-G scales to 48 FXS/FXO ports in one unit, and the MX120G to 96, so several buildings can often be served from a single platform depending on distance and port count. A distributed design with local survivability may be better for reliability even where the port count would allow centralisation.

Is there a per-user licence on the PBX?

OM-series platforms license by device rather than by person, so a building full of legacy analogue phones is not billed by headcount the way per-user models would treat it. This is often the single largest difference in total cost between a hardware-based and a software-based approach to this project.

Conclusion

POTS replacement is a port-mapping problem wearing a migration costume. Count the endpoints, split them into FXS and FXO, decide honestly which devices you are converting, and then size on concurrency rather than on wall sockets. Use the built-in analogue ports on the OM series when the population is small, add an MX-series gateway when it is large or spread out, and put a local survivability exchange at any branch where a cut cable would be a real incident. Budget the cabling, the parallel line rental and the endpoint replacements, not just the boxes — and then draw the failover path for each site on paper before anything is ordered.