Adding a handset to an expandable cordless telephone system is a great way to create more extensions without having to use more phone jacks. Expandable phone systems are tremendously varied in their capabilities. Some support multi-line systems, while others simply give you a way to access one phone line with multiple handsets spread around the house. Whether you want to add a handset to your home or business phone, it's simple enough to do.
Confirm that the cordless telephone system your are currently using is expandable. Expandable cordless phone systems come with a base station and at least two handsets. The telephone system should say "expandable" right on the base.
Make note of the brand and model number of your expandable system. Additional handsets need to be compatible with the original system.
Decide how many extra handsets you want to add and double check whether your system will support that many additional units. If you don't have the original users' manual, most manufacturers' websites have them available for download.
Purchase the desired number of compatible accessory handsets.
Plug the AC adapter to the handset's charger base into an electrical outlet and place the charger on a flat, level surface. In order for the handset to work, this charger must remain plugged in.
Install batteries in the handset and place it in the charger, with the keypad facing outward. Confirm that the phone is charging by looking for the "charging" indicator light. The light will remain on as long as the handset is charging.
Allow the handset to charge for at least 10 hours prior to using it for the first time.
Place the accessory handset in the base station of the system. For some simple systems this may be all you need to do to register the new handset. If so, the LCD screen will read "Registration Complete."
Press the "menu" button on the base station, scroll down to "register handset" and press "ok."
Press and hold either "#" or "OK" (depending on the brand) on your handset until your hear the indicator tone. At this time the LCD should confirm the handset is registered.
Clear the registration information from the base if you want to register your handset to a new base.
Hold  and [#] down on the base for a minimum of 5 seconds or until the "System Reset" menu appears.
Choose the "deregister" option from the menu and scroll down to the handset you wish to remove.
Press "OK" to de-register the correct handset. Press "yes" when you are asked if you are sure, and wait for the confirmation tone.
Hold  and [#] down on the phone for a minimum of 5 seconds until the "System Reset" menu appears.
Choose the "base unavailable" menu, select "yes" and press the "OK" button.
Purchase a 5.8GHz cordless phone system if you are running DSL on your computer. The 2.4GHz systems run on the same frequency as the DSL and can cause interference.