The title says enough, is a 56k fax modem different from a regular dial up modem? Can they be used interchangeably?
Dial-up modems may be at different speeds, e.g. 14.4 kb, 56 kb etc. These modems may also have the capability to send fax documents, e.g. to medical services or restaurants that still use fax. A 56k fax modem is a particular specification.
This example claims to support "USB 56K Voice Fax Data External V.90 V.92 Modem" standards. N.B. telco services in various countries have differing standards, so be sure the modem works in your location.