I have registered a new portable handset to my base station but one of my already registered handsets does not work anymore?

The new registered portable handset is automatically allocated to a free internal number ("1-6").

When all of these six internal numbers have already been allocated to a handset, the new portable handset will automatically get the internal number "6". The portable handset allocated to that number before will be de-registered.

You can manually change the allocation of a new portable handset to a different internal number (or name) afterwards if you want.