Yep, whenever someone rings my house (not sure if I call someone else..) the internet dies, when they stop the call, internet comes back, no router restart helps...
all the sockets have micro filters so this is strange, and here is the twist!
I recently fitted a BT I-plate, and it worked wonders (doubled the download speed instantly, after 3 days the upload doubled, and after about 10 days download was at 3.5mb (original was 1mb(ish) but that's now gone since I restarted the router twice very quickly..)

so, anyone have any ideas? its confusing the hell out of me :S

Sounds like one of the filters is probably faulty, or it could be a cabling fault to one of the extensions.

Unplug absolutely everything from every socket except the router attached at the master socket, and see if ringing the house cuts it off. It's a process of elimination - just keep plugging stuff back in until it breaks.

is the bell wire still fitted within the BT phone line main connection housing and any other secondary connections that you have
if so, please try disconnecting the bell wire from all BT connection boxes ...

I had a similar problem earlier in the year. It was eventually diagnosed (by an openreach engineer) as a defective master socket.
Do you have any interference on your phone line when making or receiving calls? Have you tried running a line test (details in the phone book or initiate one from the BT website).

Line faults aside, if you are on a 21CN enabled line then your slow down is likely to be due to the BT line management system following your router restarts. The 21CN system seems to be a lot less tolerant of router restarts that the older system and will throttle the connection very quickly and it will then take several days to recover.