how can fix my bt synergy 4500? call waiting?

BT synergy 4500....
Call waiting.... In the user manual it says that when my receiver beeps to indicate another call coming in, I should be able to see the number or name if I have it in my phone book for 10 seconds but none of my 5 handsets do this and it is very annoying as I had this function on my last phone bt freelance and found it so useful....I have tried the helpline and they were not very helpful