G.711
Fax Pass-Through
Fax audio travels in an ordinary G.711 voice stream. It can work on a stable network, but packet loss, jitter and timing changes can disrupt transmission.
Boardless Fax over IP
Fax Over IP Software with SIP/T.38 — No Fax Boards Required
FaxFacts runs entirely boardless over SIP and T.38, so fax capability can be virtualized, hosted, or run alongside an existing VoIP PBX without dedicated fax hardware.
Works with standard VoIP PBX vendors including Cisco, Asterisk, Alcatel-Lucent, Quintum, Avaya and ShoreTel, and can virtualize fax infrastructure even where no VoIP environment exists yet.
No fax boardsFully boardless, virtual or hosted deployment.
T.38-firstPurpose-built for reliable fax transmission.
~45 kb/s per callLighter than G.711-mode FoIP or POTS.
NAT supportedWorks behind standard network address translation.
FoIP basics
G.711 fax pass-through vs. T.38 fax relay.
Both carry fax over an IP network, but they handle the fax signal differently. Confirm which modes your PBX and provider support.
T.38
Fax Relay
T.38 carries fax data in a format designed for IP networks, with mechanisms to accommodate timing variation and packet loss when both ends support it.
How a call negotiates T.38
From SIP offer to a reliably delivered fax.
Carrier interop is a real constraint, so the negotiation is designed around how ITSPs actually behave.
Call initiates
The engine offers the call to your SIP provider or PBX, typically starting with standard G.711 audio for maximum carrier compatibility.
T.38 negotiated
A re-INVITE upgrades the session to T.38 once both sides confirm support, avoiding the high failure rates some carriers see when T.38 is offered upfront.
Timing protected
T.38 absorbs network jitter and congestion by queuing data so it can catch up, instead of letting the call fail outright.
Data recovered & delivered
Built-in redundancy recovers missing, dropped or out-of-order data so the fax completes reliably.
Three ways to deploy boardless
Choose the deployment that matches your infrastructure.
Boardless & Hosted
Runs anywhere on all-virtual ports using software such as Commetrex BladeWare, with a direct SIP/T.38 session to your ITSP. DNIS/DID and Caller ID are available inbound and outbound since the call is fully digital.
Boardless with Direct SIP/T.38
Runs on your local network — virtual or physical — separating fax traffic from internal VoIP/PBX systems. Works with a cloud SIP provider and runs easily side by side with an existing PBX.
Boardless Behind a PBX
Runs like a traditional fax server, using extensions off your existing PBX ports — a drop-in replacement for an older board-based fax solution.
Getting the network ready
What FoIP needs from your network.
Most FoIP reliability problems trace back to a handful of network requirements.
- 1A fixed, externally visible static IP. FoIP will not work reliably with a dynamic IP address.
- 2SIP signaling on port 5060, plus an open RTP media port range sized to the deployment.
- 3NAT support, so FoIP can run behind standard network address translation.
Modernize legacy fax lines
Replacing POTS or legacy fax lines?
FaxFacts can move fax workloads from traditional telephony onto boardless SIP/T.38, running on-premises, virtualized or in hosted infrastructure you control. The right route depends on your PBX, carrier and incoming number setup.
Plan the migration around your network.
Review your provider's T.38 support, inbound routing, port capacity and fallback needs before moving a production fax number. Discuss your architecture with Copia.
Fax over IP FAQ
Questions about running fax over SIP and T.38.
Do I need special fax hardware for FoIP?
No. FaxFacts runs entirely boardless over SIP and T.38 using software such as Commetrex BladeWare, SR140 or TE Systems XCAPI — no in-computer fax boards required.
Does FoIP work with our existing PBX?
Yes. FaxFacts works with standard VoIP PBX vendors including Cisco, Asterisk, Alcatel-Lucent, Quintum, Avaya and ShoreTel, and can run side by side with an existing PBX or take fax traffic off it entirely.
Why does fax need T.38 instead of a normal voice codec?
G.711 pass-through transports the fax audio in a voice stream and can be sensitive to packet loss and timing variation. T.38 fax relay transports fax data in a format designed for IP networks when both sides support it.
How much bandwidth does T.38 fax need?
Traditional digital fax needs roughly 56-64 kb/s, and a G.711-mode FoIP call needs about 80-100 kb/s. T.38 mode needs only about 45 kb/s per call.
What network requirements does FoIP have?
A fixed, externally visible static IP address, SIP signaling on port 5060, and an open RTP media port range sized to the deployment.
Can FoIP run without any existing VoIP environment?
Yes. FaxFacts can virtualize fax infrastructure even where no VoIP environment exists yet, deployed boardless and hosted with a direct SIP/T.38 session to an ITSP.
Related fax solutions
FoIP is one deployment option on the FaxFacts platform.
Validate your network for FoIP
Tell us about your PBX and network setup.
Share your VoIP/PBX vendor, line count and current IP configuration. Copia can help determine the right FoIP deployment and validate the network before you go live.
Talk to Copia
Send a quick question about Fax over IP.
Add a few details and a Copia fax specialist will follow up — no obligation.