Why does my BT Freelance XD 550 have no dial tone?

On answering a call today I received the tone for an engaged line. Since then I cannot get a dial tone on any of the 3 handsets for my BT Freelance XD 5500. I have checked all connections and everything is fine. I've plugged in the phone I used to use and this does give me a dial tone and calls out and receives calls no problem. I've switched off and on, disconnected power but still no dial tone. Anyone had the same problem and got any answers?