Using phone decreases noise margin by 3dB

My set up is:
BT cable comes inside to master socket via A and B connectors on master socket.
NTE2000 filtered faceplate on master socket.
No other extensions or wiring.

My line atten is 62dB, (netgear dg834G), and the noise margin is 6db.
I'm about 3.5km by road from the exchange and BT have capped my line to 1MB due to sync losses.
Line checkers suggest I should get 3MB on this line.

It seems that when I use a phone, either corded or cordless, the noise margin drops 3db - get a 6db drop if i use a normal adsl splitter on the master socket. It seems this can be enough to make the router drop out.

I've also noticed that, if no phone is connected on the line, then the router will sync with the exchange but no ip address is assigned.

Any thoughts?

I wasn't clear about the 6 dB drop - is that using a microfilter rather than the filtered faceplate ?

Try using a microfilter in the phone port of the filtered faceplate ie two filters in series.

Might be a line fault if it is independent of what you do at your end.