Not sure about the best software for it, but you can run a phone line into a analog modem, if it is voice enabled, should be able to provide caller ID, fax and answering machine voicemail (multiple message boxes) capablity even while online with DSL. Most voice enabled Modems come with some sort of software (usually crapy, limited versions) but once you know the idea works you can upgrade.
I can highly UN-recommend Communicate! (01 software), even free it's is overpriced
Something from the symantec/Winfax family might work. [/u]