GTT

start by locating your BT master socket--usually in your downstairs hallway, it will have BT marked on it.
unscrew both the screws and remove the front-- which will reveal another BT socket underneath,
plug in a new microfilter into this socket and both your phone and your broadband cable.

you have basically disconnected the rest of your phone sockets around the house. If you still have the same problem it is either.
your phone line or your broadband modem.

next step after that it to borrow a friends modem

if you have the same problem again, it is your phone line.
check with BT you dont have any extra line services enabled-- i seem to recall call waiting used to cause problems with broadband connections when phone calls came in.


once you have done all this call your ISP and they will escalate it to BT for an engineer to attend.
if your ISP is an Indian call centre then it may be easier ringing BT yourself...

hope this helps.