I googled FAX PHONE SWITCH and got a few nice enough hits, I think.
And I'm sure your fax and phone equipment supplier has them or can order them.
We have an all in 1 MP830 Cannon printer and fax machine. In year's past it got a lot of use but lately, its use has been minimized. At times we still need to use the fax but rather than paying for two phone lines, we are interested in trying to hanle the incoming phone calls and faxes on the same telephone line. In the past there was some type of telephone switching device that detected phone calls versus faxes and avoided the problem of the fax taking over the phone line. Does this still exist and is it effective?