We have a BT diverse base station with answer machine and a Philips extension unit - so that combination works. As I understand it, you should be able to do this for all manufacturers, but I have also been told you can't by an assistant in a well-known electrical retailer (who I didn't believe). I recently looked for a single extension without success - they seem to be less popular than they were - maybe the sets are so cheap everyone just buys a whole new set (base plus extensions) if they need more phones.