Perhaps you don't have UDUB properly defined
You do not state your provider but I assume this work from your phone and then your call diverts are correctly set (you should be able to define the call diverted to any number from a call reject button, at least if you are on AT&T/T-mobile. It is called User Defined User Busy supplementary service.
I'm assuming the BT puck does not signal this properly over the BT connection to the phone, or at least it does not work for your phone.