First try a (preferably corded) phone into the test socket inside the master, and do the Quiet Line Test. 17070 Option 2. It should be silent.
If it isn't, report a voice fault, (noise on the line with the phone in the test socket), to whoever you pay line rental to. Do not mention broadband!
, or else they will just tell you to contact O2, and they cannot do anything about voice faults.
Clearing voice faults nearly always fixes broadband problems. Looking for broadband faults when there is a voice fault is a waste of time.