Have you reset the handsets and base?
To reset the Phonebook, Caller ID, Redail List and registered handsets,
you have to first reset the base, then the registered handsets one
after another:
Press the dot (shift function) placed between the A and V keys followed
by 70008 and then OK (in the middle of the wheel) to reset the base.
Press the dot followed by 70009 and OK to reset each single handset.