Many analogue phones are fitted with an RJ11 connector. It's easy to insert an RJ11 plug into an RJ45 socket and it should work for a voice connection if the other end of the link has the right wiring or the correct adapter. The right way to use an RJ11 connector in a data cabling system is use an RJ11 to RJ45 adapter.
RJ11 plug to RJ45 socket adaptor.
4 Wire version most frequently used.
Commonly used to to connect patch cable to RJ11 broadband socket.
Tailed dongle type adaptor.
Adaptor with Cable 200mm long cable length 150mm ( approx.)