One reason for having fax detect is to... detect and handle faxes. I get about one fax call per year on my voice line. When it was my landline, I had a voice modem which doubled as an answering machine. The modem would detect the fax tone, though, and quietly accept the fax. Otherwise, the phone would ring every five minutes ad infinitum.
There really ought to be a way to tell the calling fax machine not to call this number again. Perhaps a touchtone would work, although it is probably way too late in the game to mandate such a requirement at this point.