David,
There could be several reasons why your base is not registering including:-
- Faulty battery which provides enough power to run the phone itself, but is to weak for transmission.
- You have 5 phones already registered with teh base even if you only actually using two.
- The phone itself has a fault.
Given that you have said that the phone appears to discharge batteries faster than normal tends to lead me to believe that it is an electronic fault with the phone.
I would suggest updating it to a Mark II (modified) phone as discussed in other threads, PM me for information.