Fax over IP software that supports SIP protocol

Question

There are a number of Fax over IP services like eFax, which often provide faxing software that you can use only with that service.

Also, there are wonderful SIP softphones like Counterpath Bria, which unfortunately do not have faxing functionality.

However, I would prefer to use faxing software that can connect to an arbitrary VOIP service provider using SIP protocol. I need that only for outgoing faxes, because I my VOIP service provider already has a very satisfactory incoming fax solution.

I have searched in the Internet for such Fax over IP software, but did not find anything substantial.

Here is some additional information:

• What operating system do you want the application to run on? Windows 8.1 or OS X Yosemite
• What budget do you have? That depends on the software. The software would be for personal use.

Answer 1

Try searching for "T.38 softphone". List of these softphones is quite short and AFAIK none of them is free. I've tested "Fax Voip Softphone" and Kapanga some time ago, Kapanga is available for free for personal, non-commercial and non-revenue generating use.

FAX over non-T.38 stream is less reliable - some can accept that, but results are often bad. Another problem is that not all operators support T.38 - from my tests (with PABX and POTS FAX machines) only one of few local operators supported it. I think this service was mostly replaced by mail-to-fax and this is what you should be looking for - you would need just mail client and optionally some kind of scanner.