When we use the term modem in describing the fax process, we mean a standard faxmodem.
A standard faxmodem is usually referred to as a 56K modem or a Dialup modem. It is not a DSL or Cable modem, although you can have both a standard faxmodem and a broadband modem. For more information about this see the Faxing with DSL or Cable topic.
The faxmodem must be a local faxmodem, that is to say it cannot be a shared network faxmodem.
The telephone line must be a regular phone line with dialtone. Basically, if you cannot plug a fax machine into it and send faxes, then you also cannot use it with a faxmodem.
Some modems are built better than others. We have a list of problem modems
It is important to distinguish between the fax part of faxmodem and the modem part of faxmodem. These are really two separate things. The modem portion or the hardware (usually used for dialup internet access) might work just fine, and the faxing part of the hardware can still be problematic. Our theory is that modem manufacturers put a lot more quality testing into the modem aspect, and a lot less into the faxing aspect. This is how you get a situation where the modem works just fine when accessing the internet, but fails when trying to send a fax. It is because it is really two different things, and two separate protocols.