I have a BT 8600 Call Guardian phone, which has worked satisfactorily for years. Suddenly outgoing calls don't work. I get either a 'number unobtainable' tone, or a 'number not recognised' message, or silence. Incoming calls seem fine. I've tried the usual switching off and on again. Any better ideas, please/
Have you been moved to digital voice?
If so you need to plug the phone into the green socket on the back of the Smart Hub 2 or a Digital Voice Adapter
Does the home page of the hub manager show your number in the box labelled 'Phone' or 'Not Configured'?
Its quite possible that its an exchange fault, where its not decoding the tones properly. The exchange line cards are very old, and its not unusual for them to develop a fault, which is one of the reasons the old PSTN network is closing down.
If you ever re-contract or change your BT package, you will be moved onto BT Digital Voice, which does not use the equipment in the exchange.
You need to prove its not the phone first, before reporting it to BT, to save a possible charge.
If its an exchange fault, then its fairly easy for an Openreach Frames Technician to mode your line to a different equipment number.