FAX-TEL (sometimes listed as FAX/TEL) is a switch that can change a fax machine from “fax” mode to “telephone” mode.
Switching a FAX-TEL switch to FAX will make the fax operate as a fax machine. It will answer the phone line when it rings, and expect that a fax is to be received.
Switching a FAX-TEL switch to TEL will make the fax operate as a telephone. If the line rings, it will not receive the fax. The line can be manually answered and “receive” pressed on the fax machine, however.