can i change the message about this "it will ask the caller to leave a brief message saying who they are and then press the hash key on their phone"
( i have not the bt 8500 i don't know the exact original message)
The BT8500 manual (page 19) says:
"You can personalise the BT Call Guardian greeting by adding your name to the Announce message."
So if you want to do more than that the answer is No.