Faximum Mfax is a low-level real-time fax utility designed to provide basic faxing capabilities for software developers at a price-point that has not been seen before in commercial UNIX fax software.

For only $495 in single quantities (and agressive discounts for developers bundling this software with their own products) you can license a package that will enable you to:

support most Class 2 and 2.0 fax modems (see complete list for details)
run on all platforms support by Faximum
fax TIFF-F and/or ASCII files
The developer is responsible for scheduling the fax requests and handling retries...the mfax utility attempts to send the fax as soon as it is given the details of the request and returns the success or failure status as soon as the call is completed.

Conversely, mfax can be configured to listen for incoming calls and receive faxes. The developer can then arrange for the received fax to be printed or stored as desired.