I'm confused. If the bare ends are to go into the telephone it needs a BT plug on the other end rather than an RJ11.
Such things are called
Line Cords and are widely available from e-Bay and other places such as
https://www.britphone.net/product-ca...n-line-cables/
If it really IS RJ11 to bare ends the easiest thing would be to get a cable with a plug at both ends and cut one off.