Borrow a normal corded phone and try that plugged into your master socket. If that works you have a problem with the phone and it'll probably be simplest to replace it. If the test phone doesn't work, you have a problem with the phone line so contact your service provider (and God help you if it's Talk talk)
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cordless phones.