If you haven’t already you need to connect in your master socket test port , this will eliminate any possible internal issues ( extension wiring, sockets etc ) and also , preferably with a corded phone , dial 17070 and take the quiet line test, any ‘noise’ , report the line noisy , this only applies if your telephony isn’t DV , but as already stated it could simply be that your connection is affected by crosstalk, which becomes more likely the more users on the FTTC cabinet , this isn’t a fault.
