Hello my name is***** look forward to assisting and am very sorry to hear that you are having this problem. You can have a max total of 6 handsets registered to this unit. The model number you would want for additional handsets is KX-TGCA20. The only places I know of that you can still find them is Amazon and eBay they normally cost around $20 each: Click Here and here. If you have any questions at all click on reply to expert and let me know. If not please don’t forget to leave a positive rating for my help - it is greatly appreciated. Please note you are rating me only not this site or your Panasonic.