My telephone line is dead, but my service provider has run a test and say it’s fine, how can that be?

The line system that BT (or your service provider) uses can be quite inaccurate in flagging up fault conditions. Even though when you pick up your telephone there is no dial tone, when a test is run from the exchange to your property, the system can still see the master socket (this is what the test system is looking for) and therefore give a clear line test. I can test the line back towards the exchange to give a more accurate fault location and advice if it is within your curtilage, or proves to be in the BT network, in which case I can assist getting a fault report raised for BT/Openreach.