Doing Fax Over a VoIP Phone Line

Q: How can I send and receive faxes over a VoIP phone line, reliably?

While it is quite possible to use voice-over IP (VoIP) providers to send and receive faxes, many users have reported problems with reliably. Some fax transmissions will go through OK, but a percentage will fail. If faxes are an integral part of your business that you use every day, you might want to keep a landline for faxing or use an Internet fax service. You and your customers don’t need the frustration of failed faxes.

The problem is that VoIP standards are designed for low-bandwidth, loss-tolerant voice calls and not for 33.6 Kbps fax transmissions where data loss aborts a transmission. Even at a fax machine’s lowest speed, you’re trying to transmit at 9.6 Kbps down a VoIP channel of 8 Kbps. Packet loss is inevitable on any IP connection. You can still understand a voice call, but lost bits in a fax call are disastrous. That said, there is great demand for fax over VoIP so VoIP service providers will let you try it as often as you wish.

Both Vonage and AT&T CallVantage offer fax services with VoIP. Here are a few recommended steps:

1. Set up the call using a 64 kbps codec. If you’re using Vonage, the dedicated fax line will use this rate. AT&T CallVantage’s fax line uses this rate by default as well.

2. If you can’t get access to a 64 kbps channel, set the BAUD rate on your fax machine to 9600 bps or lower.

3. Set your resolution to standard.

4. Turn off ECM.

Eventually, reliable VoIP fax in real-time (not store-and-forward electronic fax service) will be available universally. It’s available right now in the form of ITU T.38 standard fax gateways. However, these gateways are not widely deployed and if any of your fax transmission travels over a network leg that lacks T.38 support, the fax will fail. Be patient, and use regular landlines for heavy faxing in the meantime.

Do you have a question about faxing? Send me your fax question.