Make a call to
150 and report a
voice line fault. Don't mention broadband at all otherwise you will end up just chasing your own tail.
I assume you are aware how to use the
BT Line Test Facilities? Call
17070, select option
2 and listen. It should be absolutely quiet. Anything other than quietness and it is an indication of a POTS line fault that must be corrected by your telephony provider.
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100% Linux and, previously, Unix.