Hardware & Endpoints
ATA (Analog Telephone Adapter)
By Myroslav Orshak · operates licensed carrier infrastructure · Updated September 2026
An ATA (analog telephone adapter) is a small box that lets an ordinary corded phone, fax machine, or alarm panel keep working on an internet phone service, by converting between the old analog signal and modern VoIP.
How it works
The adapter has a network socket on one side and one or two standard phone sockets on the other. It registers with your phone service exactly as a desk phone would, then generates the dial tone, ringing voltage and signaling that an analog device expects. The old equipment has no idea anything changed.
That works well for voice, because the human ear tolerates the small imperfections of compression. It works far less well for machines. Fax machines and card terminals send precisely timed tones, and a compression algorithm designed for speech treats those tones as noise it can safely approximate. The result is a fax that transmits four pages and fails on the fifth.
Why it matters
For most businesses the ATA is what makes the math work when moving off traditional lines. Rather than replacing an overhead paging system, a lobby phone, a gate intercom, or an elevator phone, you put a small adapter in front of it and keep the equipment.
It also has an important limit, and it is a safety one rather than a technical nuisance. Anything on an ATA depends on your internet connection and your building's power. So an elevator phone or fire alarm line that must work during a power outage should not move onto one without a deliberate plan. That usually means a battery backup, or keeping a traditional line for that one circuit.
Fax needs the uncompressed G.711 codec, and even then works better through T.38, a standard that carries the fax as data rather than as audio. If a provider cannot offer T.38, expect intermittent failures on long faxes rather than a clean success or failure.
Related terms
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Find a provider that supports analog devices →Frequently asked questions
Will my fax machine work through an ATA?
Sometimes, and unpredictably, which is worse than a clean no. Ask the provider for T.38 fax support and set the line to G.711 without compression. If faxing is genuinely important to the business, an internet fax service is more reliable than any adapter.
Can I keep my alarm or elevator phone on VoIP?
Technically yes, but check the rules before you do. These circuits are often required to keep working during a power outage, and an adapter needs both electricity and internet. Many businesses keep one traditional line for safety circuits and move everything else.
How many phones can one adapter handle?
Most have one or two phone ports, each acting as its own line with its own number. Larger units exist with eight, sixteen or more ports, which is the usual way an older building with analog wiring in every room gets moved across in one step.
Is an adapter cheaper than replacing the phones?
Nearly always, at least at purchase. An adapter costs a fraction of a new desk phone and keeps equipment people already know how to use. The trade-off is that analog phones cannot show a directory, transfer calls easily, or use most features of a modern system.