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.