What is T.38 in simple terms?

T.38 is a protocol that lets fax machines communicate over IP networks (the internet) instead of traditional phone lines. It converts fax tones into data packets in a fax-aware way, so the transmission stays reliable even over a network designed for voice calls.

What is the difference between T.38 and G.711 for faxing?

T.38 converts fax signals into fax-specific data packets with built-in redundancy. G.711 treats the fax signal as uncompressed audio and sends it as-is. T.38 is more reliable under packet loss (tolerates 10%+), while G.711 is simpler but fails at even 1% packet loss.

Does T.38 work with all VoIP providers?

No. Both the sending and receiving sides of the connection must support T.38. If any network segment in between doesn't support it, the fax falls back to audio passthrough — reintroducing all the usual [VoIP fax problems](/blog/voip-fax-problems/).

Is T.38 the same as fax over IP?

T.38 is one method of fax over IP, but not the only one. Fax over IP (FoIP) also includes G.711 passthrough and store-and-forward (T.37). T.38 is the most reliable real-time method because it was designed specifically for fax, not voice.

Do I need T.38 if I use a cloud fax service?

No. Cloud fax services like [mFax](https://mfax.to) handle the protocol layer for you. You upload a document and the service manages the transmission — you never need to configure T.38, G.711, or any VoIP settings.

T.38 Fax Protocol Explained: What It Is & Why It Matters

T.38 is the protocol that makes fax work over IP networks. Learn how T.38 fax relay works, how it compares to G.711 passthrough, and when you actually need it — plus a simpler alternative that skips VoIP entirely.

Frequently Asked Questions

What is T.38 in simple terms?: T.38 is a protocol that lets fax machines communicate over IP networks (the internet) instead of traditional phone lines. It converts fax tones into data packets in a fax-aware way, so the transmission stays reliable even over a network designed for voice calls.
What is the difference between T.38 and G.711 for faxing?: T.38 converts fax signals into fax-specific data packets with built-in redundancy. G.711 treats the fax signal as uncompressed audio and sends it as-is. T.38 is more reliable under packet loss (tolerates 10%+), while G.711 is simpler but fails at even 1% packet loss.
Does T.38 work with all VoIP providers?: No. Both the sending and receiving sides of the connection must support T.38. If any network segment in between doesn't support it, the fax falls back to audio passthrough — reintroducing all the usual [VoIP fax problems](/blog/voip-fax-problems/).
Is T.38 the same as fax over IP?: T.38 is one method of fax over IP, but not the only one. Fax over IP (FoIP) also includes G.711 passthrough and store-and-forward (T.37). T.38 is the most reliable real-time method because it was designed specifically for fax, not voice.
Do I need T.38 if I use a cloud fax service?: No. Cloud fax services like [mFax](https://mfax.to) handle the protocol layer for you. You upload a document and the service manages the transmission — you never need to configure T.38, G.711, or any VoIP settings.