BT Telephone extension?

I have had an extension on my BT phone for 25 years. The old dial type phone fell to pieces and I replaced it with a more recent tone dial BT phone. I can dial out and talk but it doesn't ring. I have tried three others and they are the same, and they all work on another line. My extension is 2-wire; blue/white and white/blue going to pins 2 and 5. Has anybody any clues please.