When a length of telephone wire is not long enough, you can extend it by joining the ends of two pieces of wire. Unlike speaker wire that you can splice together, phone wire has a connector on each end. This wire must be joined by using a different method. To join telephone wire together you will need a small piece of hardware called an RJ11 coupler.
Plug one end of the phone wire into the phone jack on the wall.
Plug the other end of the phone wire into one end of the RJ11 coupler.
Connect a second phone wire to the phone that you want to use.
Plug the other end of the phone wire into the open end of the RJ11 coupler.
Create a longer extension by using additional couplers and telephone wire if necessary.
RJ11 couplers can be found at most electronics stores that sell telephone equipment and parts.