Hi and welcome to Beoworld!
Maybe try resetting the base using the connected Beocom 6000. Perhaps it has too many phones registered to it.
Assuming the Beocom 6000 is a Mk2 (with an INT button) enter the sequence 9#9#9* and press INT. Use the scroll wheel and choose reset base.
EDIT: I think you can also reset the Beocom 5 using the same 9#9#9*INT sequence.
