Is there any problems shown here. https://my.bt.com/consumerFaultTracking/public/faults/tracking.do?pageId=31
The first basic check is to see if you have a working phone line. Without dial tone you need to report phone fault to 151. Not broadband
You will need to test for noise using a phone, as noise on your line will cause dropouts. There is no other way to do it.
Buy a cheap (less than £10) wired phone to run a check.
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.
If there is a fault in your area showing on the link I posted then you need to wait on openreach repairing it