PC-Telephone

Description of PC-Telephone program from Softdepia.com:

PC-Telephone® is a powerful communications software application that enables you to exploit the full communications potential of the Internet and public telephone networks ISDN/PSTN. PC-Telephone turns your computer into Internet telephone, ISDN telephone, fax machine, answer phone, voicemail client, file transfer system and much more...

PC-Telephone is a "Free to try" program. You can download and try it for an evaluation period.