hwo can i register my 7100a handset to base?

we have just got 2 handsets 7100a . 1 model is base 1 and the other is now unregistered how can we fix this problem. i think i must have altered something as when we got them they had base 1 and 2 . the second phone is now unregistered and is showing base 1 as well. what can we do. thanks sam