Sandy, write to them, make sure in the letter you say you've had no response and make it clear the date you are leaving from, say you want a response in writing, makes more work for them at the very least, and they can't ignore you
Wasn't there a bloke who set up his own premium rate number, and used it as his contact no, so it cost them every time they rang, I think he made a couple of grand out of it, he listened to everything they had to say, and he asked questions...