This is a photo of what I assume must be the master socket.
http://img508.imageshack.us/i/img00002201101051717.jpg
The only way to be sure is to look inside, and while you have it open perhaps gently remove a wire.
Open it up and have a look which of
these it is. If it is a 1A, the ones with the cylindrical capacitor, then that is the master.
Your extension(s) may or may nor be connected to those connectors you can see. If they are, then gently remove any on T3 -
make sure you check the numbers! The ones that do the work are T2 and T5. T3 is the ring wire that was mentioned earlier, but cutting it is not recommended. Just disconnection.
There is definitely a noise on my line with my corded phone regardless of which phone socket I plug it, I will try and check with another corded phone.
Is your phone line rental with BT? If so, and you get a clued-up person at the other end the correct price for getting that master replaced by an NTE5 is around £25, perhaps plus Vat. If you can get that organised at that price, the engineer who does it will automatically be checking the line from your end. Maybe worth thinking about?
If you try that the "non-clued-up" may tell you it is £170 or so for a callout

. If that happens, come back here and one of us will give you the link to the BT Openreach price list, but I don't have it to hand sorry and it can take a bit of finding.
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