Does VoIP Support Faxing?

SIPcity supports faxing over IP using a virtual fax (vFax) number. A vFax number replaces a physical fax machine and uses email to send and receive faxes through our fax gateway.

T.38 fax protocol

SIPcity uses the T.38 protocol to transmit faxes over the internet. Whilst not quite as reliable as legacy PSTN fax services, our service successfully transmits approximately 95 of 100 faxes. However, consider the following scenarios which can affect transmission reliability:

Receipt and delivery

We recommend using your email client to send and receive faxes rather than a physical fax machine. This provides the most reliable experience.

High-speed fax machines

T.38 improves IP faxing reliability by reducing transmission speeds to very low baud rates (14.4 kbps). Some lower-cost fax machines cannot negotiate down from 56.6 kbps to 14.4 kbps, which causes transmission failures.

Colour faxes

SIPcity supports black and white faxing only. Whilst most colour fax machines can negotiate down to black and white, some machines cannot, resulting in transmission failures.

Fax machines with auto-answer

We cannot communicate with fax machines in answer-machine mode. The recipient must disable the fax machine’s answer-machine feature to receive incoming SIPcity fax transmissions.

Limitations and recommendations

This service is provided on a best-efforts basis. If you rely on faxing for emergency services or have critical business processes dependent on physical fax machines for both sending and receiving, we recommend retaining your legacy PSTN fax service.