How do I add an extension handset to my current config?

IF it was a used handset it was likely registered to another base. I just switched bases (1line to a 2 line base). I had to "deregister" the handsets from the old base. I am not sure if you can deregister if the old base is not present? With the old base plugged into power I hit the "function key" at the bottom left of the handset and scrolled down to initial settings. Under registration is "deregistration". I selected that and it put the handset back to its default out of the box. Then simply hit the mute button on the New base and OK on the handset. Worked for all 5 of my old handsets.

Hope this helps

If you have a Kx-tga270 phone that is registered to a base you do not have you can still register it to a new base by pressing and holding the mute button until the light flashes, the on the handset go to menu and go to inital settings register and press OK

PUT NEW PHONE ON BASE; PRESS MENU; PRESS ZERO (0) TWO TIMES; # # # ENTER A NUMBER YOU WANT ASSIGNED TO THAT NUMBER; CHARGING LIGHT SHOULD COME ON; ON YOUR EXTENSION PHONE SITTING ON THE BASE WILL HAVE AN OK COME UP IN THE BOTTOM RIGHT HAND CORNER OF THE DISPLAY. PRESS OK. HOPEFULLY YOU WILL GET ON LINE. PAGE 46 OF MY HANDBOOK FOR REGISTERING A PHONE. . TO REGISTER YOUR FIRST EXTENSION HIT 0-0 ; PRESS AND HOLD "INTERCOM" UNTIL THE CHARGE LIGHT STARTS FLASHING; THE PRESS THE OK BUTTON THAT SHOULD COME ON YOUR DISPLAY.