VoIP Fax Might Work, But There Are More Reliable Options

While it’s possible to send a fax using Voice over Internet Protocol (VoIP), it’s not a good idea. VoIP fax is not reliable, at all, and there is no IT person in the world who would recommend that you send a fax with VoIP.

Faxing is built on analog technology that isn’t compatible with digital VoIP phone systems that make calls over the internet.

I’ll explain exactly why that is and share specific problems you may run into when attempting to send VoIP faxes. If you need to send a fax without a fax machine or landline, there are simple online fax services that work far better than trying to use VoIP to send a fax.

Okay — that’s the quick rundown of key information. Let’s dig into the details.

Why VoIP fax struggles

When you make a VoIP phone call, the sound of your voice is broken down and converted into smaller digital packets that travel over the internet. Each packet contains information that identifies it within the full series of signals. Even if it arrives out of order, each piece can be reassembled into its complete original message on the other end.

Fax machines are made to send and receive the analog tones of the public switched telephone network rather than the digital packets associated with VoIP.

Unlike voice calls, fax data is highly timing-sensitive and doesn’t tolerate packet loss, jitter, or delays.

Essentially, the two technologies rely on different languages — you can’t just hook an analog device (fax machine) to a digital network (the internet) and expect them to communicate flawlessly.

Connecting an analog fax machine to a digital network requires specialized solutions, such as a fax-specific protocol like T.38. However, this depends on both the VoIP provider and the receiving device supporting the protocol to ensure compatibility.

SEE: Learn more about the differences between landline and VoIP communication.

What are the issues with VoIP fax?

VoIP technology was designed to digitize and transmit voice signals, not fax tones. Fax machines rely on analog tones that VoIP networks struggle to handle reliably. This creates several challenges when sending faxes over VoIP.

1. Packet loss and jitter

When a fax machine’s tones are digitized and transmitted over a VoIP network, the data is broken into packets. These packets can experience delays, arrive out of order, or be lost entirely during transmission. Such issues, known as jitter and packet loss, make it difficult for the receiving fax machine to reassemble the data correctly.

SEE: Discover how to fix network jitter for good.

2. Bandwidth limitations

Fax transmissions require more bandwidth than voice calls due to the amount of data involved. During periods of heavy network use, insufficient bandwidth can lead to failed transmissions.

SEE: Learn how to optimize your network for VoIP, fax, and other business communications.

3. Protocol interoperability

Fax machines must use the same protocol — typically T.30 or T.38 — to communicate effectively. If the sending and receiving devices are not aligned, errors occur, and the transmission can fail. The newer G.711 codec addresses some compatibility issues but still relies on both machines supporting it.

SEE: Learn more about using the right VoIP codecs.

4. Network firewalls

Firewalls and NAT (Network Address Translation) filtering can block or disrupt VoIP fax transmissions by stripping necessary data from the packets. Disabling features like SIP ALG can help but may expose your network to security risks.

SEE: Check out the different types of Network Address Translation and when to use each one.

VoIP fax alternatives

Use an online fax service

When you send a fax using an online service, the platform processes it through their backend infrastructure, converting it into a format suitable for the recipient’s device — whether it’s an email, a web portal, or a physical fax machine. This ensures the reliability and compatibility of a traditional fax while eliminating the need for additional hardware on your end.

The best online fax services have interfaces that are highly user-friendly, accessible via web browsers or mobile apps. While internet access is required to initiate the fax, recipients using traditional fax machines do not need internet access.

If you only need to send an occasional fax, free or pay-per-use online fax services might suit your needs. If you need to fax a check or another sensitive document, just remember that fax is only as secure as its network. Using reputable providers is always a good idea.

For businesses, subscription plans offer enhanced features, such as secure document storage, integrations with productivity tools, and support for higher volumes.

Use a fax ATA

A fax ATA (Analog Telephone Adapter) is a device that connects the fax machine to a modem or router, converting analog signals into digital data for transmission over a VoIP network.

This is a practical solution for those who want to move communication to digital channels while continuing to use existing equipment.

The key to success with this setup is choosing a VoIP provider that explicitly supports fax services as not all do. With the right provider, a fax ATA can deliver reliable performance without requiring a transition to online fax services. For businesses that already have functioning fax machines and prefer to avoid additional expenses or workflow changes, a fax ATA is an effective and straightforward option.

SEE: Check out our RingCentral review to learn more about our favorite fax-enabled VoIP service. 

Maintain separate phone lines for fax

Dedicated landlines ensure reliable fax transmission, especially for businesses that send or receive high volumes of documents. Unlike some online fax services, you won’t face page limits or need to change established workflows. This could be a good setup for mass fax broadcasting, for example.

However, this approach comes with added costs. Telecom providers typically charge $25 to $40 per month per landline, plus setup fees, which is quite a bit more expensive than a similar small business online fax service.

It’s also important to consider that the Federal Communications Commission (FCC) officially phased out the requirement for landline support in August 2022. While landline faxing remains an option for now, the future availability of these networks is uncertain as carriers shift focus to newer technologies.

For businesses with high fax volume and a need for reliability, maintaining landlines may still be worth the investment. However, those looking for long-term solutions might explore alternatives like business phone services with T.38 protocol support and online fax services.