- Fax was designed for the analogue PSTN, which Openreach is retiring as the all-IP migration completes in 2027.
- Standard voice-grade VoIP codecs compress and reorder audio, which corrupts fax tones and causes failed or garbled transmissions.
- The ITU-T T.38 recommendation defines how fax is carried as data packets over IP networks, avoiding the audio-compression problem.
- Even with T.38, success depends on both endpoints, the network path and the provider supporting the standard end to end.
- Email and managed fax-to-email (e-fax) services are common alternatives that remove the dependency on a physical analogue line.
Fax over standard VoIP is unreliable because audio compression corrupts the tones. The fixes are the T.38 fax-over-IP standard, an analogue adapter tested for fax, or moving to e-fax or email instead.
Last reviewed: June 2026
Why fax struggles on a digital line
Fax is a survivor from the analogue era. A fax machine converts a page into a series of precise audio tones, sends them down a phone line, and the receiving machine decodes those tones back into an image. The process depends on the line delivering the tones intact and in order, which the analogue PSTN did because it carried a continuous, uncompressed audio path between the two machines. That tight coupling is exactly what makes fax fragile once the call moves to an internet-protocol network.
Voice over IP was optimised for human conversation, not machine tones. To save bandwidth, VoIP codecs compress the audio and the network may deliver packets slightly out of order or with variable delay, known as jitter. A human ear tolerates this, but a fax machine does not: a compressed or reordered tone is misread, and the transmission fails, stalls or arrives as a corrupted page. The Public Switched Telephone Network is being withdrawn by Openreach with the all-IP migration completing in 2027, so any business still relying on analogue fax needs a deliberate plan rather than an assumption that the machine will keep working.
What T.38 fax over IP does
The telecoms industry anticipated this problem and the ITU-T published recommendation T.38, the standard for real-time facsimile over IP networks. Instead of treating the fax tones as audio to be compressed, a T.38 gateway demodulates the fax at the sending side, carries the page as structured data packets across the IP network, and remodulates it back into tones for the receiving machine. By moving the page as data rather than sound, T.38 sidesteps the compression and jitter that break ordinary VoIP fax.
T.38 is widely supported by business telephony providers and analogue telephone adapters, but it is not a guarantee on its own. For a fax to succeed, both endpoints, every gateway in the path and the provider's network must support and correctly negotiate T.38. A mismatch anywhere along the route can drop the call back to plain audio and reintroduce the failures. Businesses should therefore confirm with their provider that T.38 is supported end to end and test thoroughly before relying on it for important documents.
Fax PSTN switch-off options for businesses
The table sets out the main routes a business can take, with the trade-offs of each.
| Option | How it works | Key consideration |
|---|---|---|
| T.38 fax over IP | Page sent as data packets, remodulated to tones at each end. | Must be supported end to end by both endpoints and the provider. |
| Analogue adapter (ATA) | Existing fax machine plugged into a digital line via an adapter. | Reliability varies; needs testing and ideally T.38 support. |
| Fax-to-email (e-fax) | A hosted service sends and receives faxes as email attachments. | No physical line needed; check data-handling for sensitive content. |
| Secure email or portal | Documents exchanged directly over encrypted email or a portal. | Requires the recipient to accept the new method. |
For many businesses the practical answer is a combination: a T.38 or e-fax route for partners who still demand fax, alongside a move to email or a portal where the other side will accept it.
How jitter and compression break a fax
It helps to understand precisely why a digital line treats a fax so harshly, because the mechanism explains why simply plugging an old machine into a new line rarely works. A fax transmission is a tightly timed exchange: the two machines negotiate a speed, then send a stream of tones where every fraction of a second carries part of the image. The receiving machine expects those tones to arrive continuously and in the exact order they were sent. Any disruption to that stream is read as an error in the page rather than tolerated as a glitch.
Two features of a packet network conspire against this. Compression, applied by voice codecs to save bandwidth, alters the shape of the tones so the receiver can no longer decode them cleanly. Jitter, the variation in how quickly packets arrive, breaks the continuous timing the fax depends on, and packet loss removes pieces of the page outright. A human conversation absorbs all three because the brain fills in the gaps, but a fax machine has no such tolerance. T.38 solves the problem by refusing to treat the page as audio at all, carrying it as structured data and rebuilding the tones only at the very end, which is why it succeeds where plain VoIP fails.
Regulated sectors that still use fax
Fax persists in healthcare, legal, financial and some public-sector workflows, often because a counterpart will only accept documents that way or because internal procedures were written around it. These sectors cannot simply switch the machine off, but they also cannot ignore the analogue retirement. The route forward is usually to validate a T.38 path with the provider for the connections that genuinely require fax, while migrating the remainder to secure electronic exchange where the receiving organisation permits it.
Where documents contain personal or sensitive data, the method chosen must handle that data appropriately, and organisations should confirm their own compliance position before changing process. The point for telecoms planning is narrower: the underlying analogue line that fax assumes will no longer exist, so the transport must be re-established on IP terms or replaced. Leaving this until a switch-off notice arrives risks an interruption to a workflow that other parties depend on.
Alternatives to physical fax
Beyond T.38, the clearest long-term direction is to remove the dependency on a fax tone entirely. Fax-to-email services let an organisation keep a fax number while sending and receiving as email attachments, which avoids any physical line. Direct secure email or a document portal removes fax from the picture altogether where both parties agree. Each alternative trades the familiarity of a physical machine for resilience that does not hinge on a retiring analogue network.
The decision usually comes down to who the business exchanges documents with. Where counterparts have modernised, email or a portal is straightforward. Where a key partner still insists on fax, a T.38 path or a hosted e-fax service preserves compatibility while the rest of the workflow moves on. Either way, testing against real counterparties before the analogue line is withdrawn is the step that prevents surprises.
Frequently Asked Questions
Will fax still work after PSTN switch-off?
A traditional fax machine plugged straight into the analogue line will not, because that line is being retired. It can continue over IP only if the path supports the T.38 fax standard or an analogue adapter that has been tested for fax. Many businesses move to e-fax or secure email instead.
What is T.38 fax?
T.38 is the ITU-T standard for sending faxes over IP networks. Rather than compressing the fax tones as audio, a T.38 gateway converts the page into data packets, carries it across the network, and rebuilds the tones at the far end. This avoids the compression and jitter that break ordinary VoIP fax.
Does VoIP support fax?
Standard voice VoIP does not handle fax reliably because its codecs compress and reorder audio, corrupting the tones. VoIP can carry fax only when the T.38 fax-over-IP standard is supported end to end by both endpoints, every gateway and the provider. Without T.38, transmissions often fail or arrive garbled.
What should businesses do about fax when PSTN switches off?
Identify which workflows genuinely need fax, confirm with the provider whether a T.38 path is supported end to end, and test it against real counterparties. For other workflows, consider fax-to-email or secure electronic exchange. Sensitive documents must still be handled in line with the organisation's own compliance requirements.
What are the alternatives to fax?
The main alternatives are fax-to-email services, which keep a fax number while sending and receiving as email attachments, and direct secure email or a document portal that removes fax entirely. T.38 fax over IP remains an option where a counterpart still requires a fax connection. The right choice depends on what recipients will accept.
