Have you checked to see that caller display has actually been activated, any you can see the incoming number when somebody calls?
Excuse not being able to answer the problem but I also have a similar problem with BT8500 Trio!
We have spent hours trying to resolve this, the Supplier even providing a replacemnt Base Unit.
We have been through the manual several times, carried all suggested re-booting, battery removals, etc.,
The only time Call Guardian works is to an unknown landline number followed by answerphone cutting in after 4 rings.
With Answerphone 'On' known landlines and mobiles do not trigger answerphone to record a message. Unknown mobiles do not trigger Call Guardian or Answerphone. So Friends and Family cannot leave a message.
The Supplier has offered to take it all back but where does that get us? We wanted Call Guardian.
Appreciate a solution.