My Dad is deaf as a post and I lent him my �BT converse 1200� phone because it�s got an amplifier built in, you can adjust the volume on it and use it as a normal phone (but louder!) or attach a headset if required and switch between the two.
It doesn�t always get the best reviews, there�s one made by Geemark (I think) that gets better write ups but he couldn�t get on with that one.
If it�s for work I�d check with your techies that you can just plug in a normal �phone, I don�t think they work if you�re on a switchboard?! :-S