Telephone extension cords come in a variety of lengths and are inexpensive, but sometimes they aren't in the length you need. It is easy to make your own telephone extension cord and customise it to the desired length by using a specialised crimping tool.
Join two lengths of existing phone extension cord together with a modular female coupler. Plug one end of each cord into either side of the coupler.
Purchase a combination crimping tool for use with modular phone components. You cannot make a phone extension cord without it.
Cut flat modular phone cord to the length you desire, using the crimping tool. It will yield a nice, square edge.
Strip the outer covering off the modular cord. The tool will have a slot that will accomplish this to just the right length to fit into the plug.
Insert the four phone wires into a male modular plug. The black wire will be on your left as you look at the face.
Place the plug with wires into the female modular receptacle on the side of the tool. It should snap firmly into place, just as if you were plugging it into a phone jack.
Compress the handles to crimp the male plug around the wires. Remove the plug from the tool. Repeat for the other end of the extension cord.
Combination crimping tools for occasional use were available for £2 or with male plugs for around £7 as of February 2010. However, they are not production tools and should be purchased only for infrequent use.
Do not use extremely long extension cords with your phone system or you may experience line interference. Consider adding another phone jack and running shield phone or phone and data cable instead for long runs.