Knowledgebase

What is Inbound Fax?

Inbound fax allows you to receive faxes digitally through a dedicated Voyced number.

Once activated:

• The number becomes fax-only (it won’t work for voice calls)

• It operates 24/7 to receive fax transmissions

• Each fax is converted to a standard PDF and sent to your chosen email address

You can set a different email address per fax number, if needed.

What is T.38?

T.38 is a protocol designed specifically for faxing over VoIP. It ensures better reliability by compensating for the differences between traditional phone lines and internet-based voice systems.

It was introduced to handle the unique timing and signal requirements of fax, which standard voice codecs don’t support well.

More details here: T.38 on Wikipedia

Is T.38 supported by Voyced?

Yes, our Hosted IPPBX platform fully supports T.38:

• We can process incoming faxes on our side and email them to you as PDFs

• We also support T.38 passthrough, allowing your own fax machine or software to receive the fax directly

If you have a compatible fax machine or digital fax solution, you can also use your Voyced number to send outbound faxes.

Are all numbers suitable for fax?

Not all numbers support T.38.
It depends on:

• The country the number is based in

• The local carrier

• The type of number (geographic, toll-free, national, etc.)

Before you order a fax number, please check with our sales or support team. We’ll confirm whether the number type you're looking at supports inbound fax with T.38.

What should you be aware of?

1) Using a traditional fax machine?

You can use a physical fax machine by connecting it to an ATA (Analogue Telephone Adapter).
Most fax machines can't speak VoIP directly, so an ATA converts the digital signal to something your machine can use.

This setup isn’t difficult, but it might require a bit of tinkering:

• Each fax machine and ATA brand works slightly differently

• Fax standards are loosely followed by manufacturers, many add their own variations

• Fine-tuning may be needed to get things working smoothly

We can provide tested ATA options if needed.

Read more: Analog Telephone Adapter on Wikipedia

2) Faxing over VoIP isn’t always perfect internationally

This is important to understand.
While T.38 improves reliability, fax over VoIP is not guaranteed to work 100% of the time, especially when faxing across borders.

That’s because:

• Fax support is not implemented as the standard is set across all VoIP networks and hardware providers

• Some networks may not support fax at all

• Others may use incompatible implementations of T.38

• There could also be mismatches between fax machines, even if the network is fine

Faxes are very sensitive to timing and signal quality. On less-than-perfect connections, especially over long distances, small delays or errors can cause a transmission to fail.

These issues can happen:

• Between the fax machines themselves

• In the networks the fax is routed through

• Or due to codec or timing problems along the way

So yes, faxing over VoIP usually works, especially with T.38 and local numbers.
But it's good to be aware that it may not be 100% reliable, particularly for international faxes.

If faxing is critical for you, we recommend:

• Testing your setup before committing

• Using numbers with confirmed T.38 support

• Asking us for help choosing the right setup and hardware

Need advice or help getting set up?
Just let us know, we’re here to make it work.

Last updated: 01/04/2025

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All information provided is copyrighted and purely informative in nature. It is maintained by Voyced with the greatest of care using reliable sources only.
Given the fast evolution of the telecom sector, Voyced does not offer any guarantee about the accuracy and completeness of the information offered. 
We explicitly do not give anyone permission to use this information in any other way than to inform themselves about what (legal) requirements there may be and they need to comply with when registering Services and/or DIDs with Voyced.