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Fax over VoIP, also called fax over IP (FoIP), is fax communication carried across a transmission path that includes an IP network. A conventional fax machine can sometimes work on a VoIP connection, but success depends on whether the provider, adapter and receiving endpoint support a compatible fax method—not simply on whether ordinary voice calls work.
What is fax over VoIP?
Traditional Group 3 fax machines send fax signaling and data using modem tones. With fax over VoIP, part of the path between fax terminals uses an IP network, such as the internet. The International Telecommunication Union’s Recommendation T.38 defines procedures for real-time Group 3 fax communication over a path that includes an IP network.
That definition describes a communication method, not a guarantee that a particular internet phone service or device will support it. Compatibility must be checked for the actual service and equipment.
How does a fax call travel over an IP phone connection?
There are two distinct approaches identified in the Internet Engineering Task Force’s RFC 6913:
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| Method | How it carries fax | What to confirm |
|---|---|---|
| T.38 | Uses a fax-specific transport for Group 3 fax over an IP-including path. | The provider and the relevant endpoints must support T.38 and handle the call accordingly. |
| G.711 passthrough | Carries fax data as audio using the G.711 codec. RFC 6913 describes μ-law or A-law audio media for this mode. | Confirm that the service and endpoints support the required G.711 passthrough configuration. |
These are protocol distinctions, not a ranking of reliability. The cited standards do not establish comparative success rates for specific providers, adapters or fax machines.
Can you use a conventional fax machine with VoIP?
Possibly. A conventional fax machine connected through an analog telephone adapter (ATA) may be able to send and receive fax over an IP phone service, but the adapter category alone does not establish compatibility. Check the exact fax machine, ATA and VoIP service together.
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- Supports 2 SIP profiles through 2 FXS ports and a single 10/100Mbps port
- TLS and SRTP security encryption technology to protect calls and accounts
- Automated provisioning options include TR-069 and XML config files
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- Supports T.38 Fax for creating Fax-over-IP
Before setting up the connection, ask the VoIP provider and ATA manufacturer:
- Whether the service supports fax calls, and whether it supports T.38, G.711 passthrough, or both.
- Whether the specific ATA model supports the provider’s fax method.
- Whether the provider’s SIP call routing and endpoint configuration can preserve or negotiate fax capability for the call.
- Whether support applies to both sending and receiving with the fax equipment you intend to use.
ITU-T T.38 defines a transport procedure; it does not require consumer VoIP services to offer it. RFC 6913 describes SIP capability signaling, not a catalog of current products or a certification of interoperability.
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- Multi-Party Conferencing: Supports 3-way voice conferencing for enhanced communication capabilities
- Reliable Failover Protection: Failover SIP server automatically switches to secondary server if main server loses connection
Why might a fax machine fail over VoIP?
A fax call may not be handled as a fax from the start. RFC 6913 explains that SIP fax communications are handled in a “voice first” manner: a call can begin as a voice call and switch to fax handling after connection. If the call reaches an endpoint that cannot handle fax, or if the capability is not advertised or configured correctly, the switch may fail.
Other compatibility gaps can arise when the provider, ATA and fax machine do not support the same transport method. Ordinary voice calling working on the connection does not by itself confirm that fax signaling will work.
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What should you do if faxing does not work?
- Identify the connection path. Note the fax machine, any ATA in use, the VoIP provider and how the receiving fax service is reached.
- Ask the provider which fax transport it supports. Request a clear answer about T.38 or G.711 passthrough for your service and call routing.
- Verify the ATA and fax machine configuration. Check with their manufacturers whether the exact models support the provider’s method and whether fax-related settings need to be enabled.
- Ask the provider to check SIP fax capability and routing. Because a call may start as voice and switch to fax, confirm that the endpoint handling the call can support fax when the switch occurs.
- Test the full combination. Confirm sending and receiving with the actual service, equipment and destination types you plan to use; standards support alone cannot verify a specific setup.
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