voice/fax/data modem

Not sure on who makes one, as the list of modem vendors is going smaller.

But, if it you need it to answer both voice and fax, there is a problem.  The modem cannont distinguish between the two - all it knows is that there is an incoming call and will answer - and if you have the fax program running it will assume it's a fax program - if you have the voice running it will assume it's a voice program. 

Same way if you have an external voice answering machine (by the way the best way - the PC makes a poor telephone or answering machine) and the PC connected to the telephone line, neither the PC nor the Answering machine will be able to distinguish what is what.  Distinctive rings will not work with the PC either.  The only to get both ont he same line (and have the PC active) is to have a separate "call director" type device that can distinguish between an incoming voice and fax telephone call and route it to the appropriate device (e.g. voice tothe voice telephone/answering machine and fax to the PC).