Last updated · Written for practice managers and office administrators.
A POTS line can be replaced three ways. A cellular gateway (“POTS-in-a-box”) or a VoIP adapter keeps an analog jack for equipment that must stay, such as an alarm panel or fax machine. For a line that exists only to carry a fax number, an online fax service is simpler: the number is ported, faxes arrive by email, and no line or machine is needed.
Why POTS lines need replacing
Plain old telephone service runs on a copper loop from the carrier’s central office to your building. Carriers are retiring those loops (the legal definition is in 47 CFR § 51.325) and discontinuing the services on them with FCC authorization. AT&T already holds authority to end business and residential POTS in 39 wire centers on or after July 14, 2026 (FCC DA 26-459). Under 47 CFR § 63.71 your carrier must notify you in writing before it stops your service. The overview explains the stages; this page is about what to put in the line’s place.
The three replacement options
| Option | What it is | Pros | Cons | Best for |
|---|---|---|---|---|
| Cellular gateway (POTS-in-a-box) | A box with a mobile radio and an analog phone jack. Your device plugs into the jack; calls travel over the cellular network. | Works without wired internet. Many units include a backup battery. Keeps existing analog equipment. | Needs cellular signal at the device location. Fax and alarm signalling are converted to a digital call, so support varies by unit and must be confirmed in writing. Adds hardware plus a wireless plan. | Alarm panels, elevator phones, sites without reliable internet. |
| VoIP analog telephone adapter (ATA) | A small box that turns your internet connection into an analog jack. The line is a VoIP subscription. | Cheapest hardware. Uses the internet you already pay for. Keeps the fax machine. | Fax over VoIP is fragile: it needs T.38 or a very clean G.711 path end to end, which you do not control. Dead in a power or internet outage. | Offices that must keep a physical machine and can accept occasional failed pages. |
| Online fax service (for the fax line) | The fax number is ported to the provider’s network. Faxes arrive by email and in a web dashboard; you send from a browser or email. | No line, adapter or machine. Number kept. Receives faxes even when your office is offline. Delivery confirmation and an audit trail. | Only replaces the fax line, not alarm or elevator lines. Requires the number to be ported while the old line is active. | Any office whose POTS line exists for fax: medical, dental, legal, insurance, small business. |
Is POTS-in-a-box a good fax replacement?
A cellular gateway does what it says: it gives you a dial tone where the copper used to be. What it cannot do is make the mobile network behave like a copper loop. A fax call is two modems negotiating a data session over a voice channel, and the gateway has to carry that session across a cellular voice call. Some units handle it; some do not; all of them depend on signal quality at the moment of the call. For a fax line that carries prior authorizations or court filings, “it usually works” is a weak standard. Ask the vendor for a written statement about fax support, and test with the busiest counterpart you have before you move.
Is a VoIP adapter a good fax replacement?
An ATA is the cheapest way to keep a machine, and it is where most fax complaints come from. Fax over VoIP works reliably only when both ends and every carrier in between support T.38 (the ITU standard for fax over IP) or maintain an unusually clean G.711 audio path with no compression, packet loss or jitter. You control one end. The fax over VoIP guide goes through the failure modes in detail. If you choose an adapter, keep a delivery log and expect to resend.
When online fax is the right replacement
If the only thing on the POTS line is a fax machine, the line, the machine and the adapter are all solving a problem you no longer have to solve. Porting the number to an online fax service means:
- The number you print on forms keeps working; the port is what moves it.
- Incoming faxes arrive as PDFs by email and in a dashboard, with a record of every page.
- Outgoing faxes go from a browser or from email, with a delivery confirmation.
- The provider’s carrier network handles the fax negotiation, not an adapter in your closet.
- Nothing at your office has to be powered on for a fax to be received.
It is not the right replacement for an alarm panel or an elevator phone; those need one of the analog options. Inventory each line first, then choose per device. The fax line alternatives guide puts numbers on the three options for the fax line in particular.
How to switch the fax line without losing the number
- Do not cancel the POTS line. The number is portable while the service is active. After disconnection it becomes the carrier’s decision, and under 47 CFR § 52.15 a disconnected business number can be reassigned after an aging period of 45 to 365 days.
- Run a portability check. It tells you which carrier holds the number and whether it can move.
- Port the number. You will need a recent bill, the account number, the account-holder name as billed and any PIN. usfax.com charges nothing to port a number in; the losing carrier may bill a port-out fee. Details are in the porting guide.
- Test, then cancel. Send and receive a test fax on the new service, then close the POTS line.