F500 extension handset desn't store new settings. ?

Hi all. At auction I Recently bought 3x Telstra F5000 extension handsets with 1 base and 1 charger.The main purpose is to use as an intercom between home, shed, workshop and granny flat. I was hoping they would be compatable with my Telstra F5500 Cordless Phone, which has an answering machine as the base/charger. I programmed them all to my base, which is the answering machine. The plan being that the new base that I bought would serve just as a charger. All handsets work except for 1. I programmed it to be handset 3 and it works fine like this for intercom and phone for about a day. Then the display changes to handset 2 and it doesn't work at all as handset 2. I have programmed the other handsets to be 1, 2 and 4 and they work fine. I would really appreciate any suggestions that might successfully activate the handset permanently. In relocating home, I have lost the manual, which may or may not have helped. Cheers Jen