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you need to ensure the fault is not on your side of the test socket by using preferably a corded phone to check for dial tone and also to see if there is an line noise causing a problem if there is no dial tone or there is line noise when conencted to test socket then problem is outside your home and you should not be charged for engineer callout
You can run a quiet line test, think it's 17070 and option 2. See if you have noise on your line.
if you have a dial tone then you need to check for line noise by doing quiet line test as described by @B13blues
if no dial tone then report a phone fault to 151 if dial tone but noise on quiet line test then report phone fault to 151
you need to use the test socket to eliminate any internal wiring test socket is behind the removable front plate on the master socket
you need to test from test socket to be sure it is not your internal wiring that is causing the problem