I would appreciate if someone can clarify the observations I have made with regards to SNR Margin on my ADSL connection.

1. I have two phone sockets in my house.
2. The router is connected to one socket and a phone connected to the other.
3. When the phone is unplugged, my router reports a SNR Margin of 19.3dB
4. Then if I plug the phone into the other socket the SNR Margin drops to 17.4dB

When I run the speed test, it appeared that the download speed was greater when the phone is not plugged in - again, given the variablitiy of download speeds it is difficult to say whether the download speeds are definitely better in this case or not.

My questions are...

1. Why would the SNR Margin reduce when a phone is plugged in?
2. Theoretically, is it better to have a higher SNR Margin and hence go for option 4 with the phone plugged in?