I have had several instances of handsets that would not register. Just a few days ago, I found the weirdest solution with one of them, a nice yellow Mark I.
Just after I had given the base code “0000”, the registration ended with an error.
I cannot explain why, but I decided to make absolutely sure, the code was correct, so I used another handset to “change” the base code to 0000 (zero x 4). This means I used the same four zeros as old and new code.
That solved the problem.
I had another problematic handset which had the menus completely disturbed. A reset of the handset solved the problem.
As I think Guy once said, these phones are temperemental.