Fax over IP (FoIP) is the transmission of fax calls across Internet Protocol (IP) networks instead of traditional analog phone lines. Because fax machines communicate with audio tones that are sensitive to the compression and packet loss of ordinary voice over IP, FoIP usually relies on a dedicated protocol, T.38, or on carefully configured uncompressed audio (G.711 pass-through) to get those tones through intact. It’s how existing fax machines, multifunction printers and fax servers keep working once analog lines are replaced.
FoIP is not the same thing as a cloud fax service that lets users send faxes by email or web portal. Those services may use FoIP behind the scenes, but FoIP describes how the fax travels, not how people send it.
At a glance
- Carries fax calls over IP networks, typically a SIP trunk or VoIP service, instead of copper POTS lines.
- T.38 relays the fax as data with redundancy; G.711 pass-through sends uncompressed audio and is more sensitive to network problems.
- Reliability depends on the whole call path: the gateways and carriers in between have to handle fax properly.
- Common in POTS replacement projects, where fax is one of the hardest devices to migrate.
What problem it solves
Fax has outlived many predictions of its death. Healthcare, legal, government, insurance and logistics workflows still use it, and many organizations keep analog lines just for fax machines. As carriers reprice or retire those copper lines, fax has to move somewhere.
Plugging a fax machine into an ordinary VoIP line often fails. Voice codecs are tuned for speech, not modem tones, and a little packet loss or jitter that a person would never notice on a call can break a fax page partway through. FoIP addresses that by sending the fax in a form designed for IP networks, so fax keeps working without a dedicated analog line.
How it works
T.38 fax relay. A gateway, an analog telephone adapter (ATA) or a fax server detects the fax tones on a call, converts them to data packets, and sends them across the network with redundant copies so occasional lost packets don’t break the page. At the far end, another gateway rebuilds the tones for the receiving machine. The two ends switch from voice to T.38 during the call using SIP signaling, so both sides and the carriers in between need to support it.
G.711 pass-through. Instead of converting the fax, the system sends it as uncompressed audio. It’s simpler and often works on clean networks, but it has no protection against packet loss and is more prone to failures on longer or busier paths.
Fax servers and cloud fax. Software fax servers and cloud fax services receive and send faxes as files, integrating with email, document management or electronic health records. Many connect to the phone network using T.38 over SIP, but the user never sees a fax machine.
Tuning. Reliable FoIP often depends on details: disabling echo cancellation and voice activity detection for fax calls, limiting fax speed on older machines, and enabling error correction mode.
For help comparing fax options, see our Faxing solution page.
When it matters for buyers
- Copper lines are being retired or repriced. Fax is often the last reason to keep an analog line, and it needs a tested replacement.
- Moving to a cloud phone system or SIP trunks. Ask about fax before cutover, not after the first failed transmission.
- Regulated workflows. Industries that still exchange records by fax need a reliable, auditable path, plus the right contracts with the provider.
- High fax volumes or fax servers. Throughput, concurrent calls and integration with document systems matter more than at a single desk.
Questions to ask vendors
- Do you support T.38 end to end, including with the carriers you hand calls to?
- If T.38 isn’t available on a route, do you fall back to G.711 pass-through, and how do you handle failures?
- Which analog adapters or gateways do you certify for fax, and do you supply them?
- What success rate or failure reporting can we see for fax calls?
- Are fax transmissions and stored images encrypted, and where are images kept?
- Can we keep our existing fax numbers, and how does porting work?
How it differs from POTS replacement
POTS replacement is the broader project of moving every device that depends on an analog line, including alarms, elevator phones, gates and fax machines, onto a new service, often a cellular or broadband-based box that presents a standard phone port. FoIP is one way to carry fax specifically across an IP network. A POTS replacement device may use FoIP for the fax ports it serves, or it may pass fax over a cellular voice path; either way, ask the provider how fax is handled and test it before you cancel the copper line.