I have a BT8600, and also a BT8500. They're almost identical in functionality, apart from the block button and, I think, the poorer sound quality of the BT8600.
The maximum number of handsets which can be registed to the base is 5, according to the manual, but it's taken 6, including 2 of the BT8500 handsets.
Does anyone happen to know if this is likely to cause any problems with the functionality? So far, it seems fine, but I don't want to risk missing calls for the sake of an extra handset, useful though that is.
