have a look here
http://community.bt.com/t5/Phones/Crack ... td-p/55207
Some advice on checking is important.
Have you tried a quite line test from master socket
dial 17070 option 2 and listen - should hear nothing
best done with old type analogue phone digital will do but may have slight hiss
do not use home hub phone
any noise such as you seem to have will destroy your Broadband and drop speed to next to nothing
if the quiet line test shows noise when done from master socket you need to report the fault as phone line fault not Broadband
Very important to report it as a phone fault. The poor line connection impacts the internet more than the phone.