Unfortunately, Windows XP Fax has confirmed to be mismatched with frequent common modems and drivers, mainly the older ones, yet if the manufacturers declare that they are "compatible" with Windows XP. Modem inappropriateness is not obvious during installation or the early setup. It only becomes obvious only when you first try to send or receive a fax. The Fax Service will dial or will answer a receiving fax but then fail with any of a number of unhelpful error messages such as "Line is busy," "There is no answer," or "Fatal error." There will be not anything in Event Viewer to give a clue as to where the problem may lie. These errors have been almost unattainable to track down. To do so requires enabling both PSS and debug logging of fax transmissions, both of which necessitate fairly widespread registry changes. For the time being, the best advice is to try to use XP Fax only if you have a modem that is listed on the Windows XP Hardware Compatibility List and using the latest drivers.

There might be a real problem with this. For some time (multiple windows versions) the modem reply parser (a windows module) has had problems in getting the modem reply strings, and correctly parsing them. The modem logs often showed random character values immediately before the start of the reply string. This prevented proper recognition of the reply, and may cause the parser to fail, thus causing the pick of an inappropriate action. There was an argument going on at one time as to the cause of the random characters, with some blaming it on hardware, and others software. One fix that was suggested slowed down data transfer from the modem to the PC interface. The problem was occurring more often with modems that were internal, perhaps due to the faster data transfer. I've found the problem in modem logs from such things as cell phones that have a built-in modem, and from well thought of makes and models of popular serial i/o modems.