I'm in the UK. We have a master phone socket downstairs, and an upstairs extension that taps into the main socket by the use of a splitter plugged into the main socket. The extension plug is an RJ14, but the splitter's socket is an RJ11 (2 pins instead of 4).
On the downstairs socket, we can use a phone, yet on the upstairs one, we can't - we just get no dialtone. But here's the weird thing - the ADSL connection works fine when the modem's plugged into the upstairs socket. Any ideas why that might be? I thought the extra 2 pins on the RJ14 were superfluous when you only have 1 phone line.