The BT automatic systems have decided that your line is unstable at lower noise margins and so have increased the default noise margin from 6dB to 15dB
If you would like some help on improving it can you please post the stats from your router - noise margin, attenuation and speeds.
Also are you plugged into the master socket or a diffeent one
Once you have posted those we can take the investigation to the next stage
Edit - first thing is to check if you have a dial tone with only the phone connected. If you don't then report a voice fault to your phone supplier (not necessarily the same as the broadband supplier) without mentioning broadband
