All current telephones are designed for use with modular jacks, so, before extending your phone system, you will have to convert to modular jacks if you don’t already have them.

Hard-wired jacks. Telephone connections installed before 1974 are referred to as “hard-wired” because the telephone could not be unplugged. The so-called “block” attached directly to the wall or trim, and the “flush-mounted” was actually housed in the wall and covered with a faceplate that was round or rectangular.

Both block and flush-mounted connections are easily changed. Simply remove the cover with a screwdriver and replace it with the appropriate modular jack converter.

Four-prong jacks. Portable phones often used a four-prong jack mounted on a wall or trim, or a flush-mounted jack in a housing box covered by a faceplate. You can either replace the four-prong jack with a modular one or use a plug-in adapter that fits the four-prong jack.

Hard-wired phones. Hard-wired desk phones are those with a permanent connection between the line and the phone. To convert these, simply remove the phone housing and install a line cord converter.

Wall phones that are hard-wired cannot be converted. Instead, you must first remove the telephone, which will leave the wires exposed. Do not let them fall back into the wall because you will need them to install a new modular jack.