Ahh, GG, this is where some people trip up.
Just because you are so close to the exchange physically, it doesn't always mean that is the length of the cable run from the exchange to you. That could easily be over 1km! Add is assorted "joints" in the cable as lines are tapped off to others, cable degredation, bad connections in said joints and things like that then you can see where things can go bellyup.
Now, iirc there is a UK number for BT which doesn't go to Bangalore. Of course, it ain't freephone but a local call but you get straight through to UK techies. Of course, since I been over here for a couple of years I don't have that number any more but I would get onto them somehow as there could be a fault building up which could cause problems.