how do I make my hand set ring when it is not in the holder?

I have a bt synergie 3105 twin hand set cordless phone. (no answer machine) When someone calls, it rings from both slaves and from one hand set when not in the slave. However the other handset that belongs to the main slave does not. If not in its' holder it makes a pathetic noise which can only be heard if you are very close to it and there is no extraneous noise. Can you help?