Without checking for noise on your landline, there is nothing else you can do. A noisy landline is the cause of nearly all broadband issues, and cannot be detected by the normal BT line test.
The solution is to get hold of a cheap (less than £10) wired phone, and plug it into the phone socket to see if you get dial tone.
Then dial 17070 (free call) and select option 2, quiet line test, and see if there is any noise on the line.
If you do not get dial tone, or there is noise on the line, then you need to report it as either no dial tone, or a noisy line. Do not mention the broadband problem, otherwise it will end up in the wrong fault queue, and take much longer to be fixed. There are plenty of people who can fix phone faults, but not as many broadband people.
Once the phone fault is fixed, then the broadband will get better.
