You are correct, ideally you would be connecting PSTN lines which have been supplied specifically for PBX use. These lines are configured a little differently at the exchange. However, I believe you will not be provided with ADSL on these types of lines
I assume these two lines must be separate with separate numbers? Not a main line and auxiliary line on the same number? You can't normally have ADSL on these lines either.
If the line is noisy, this can upset guarded clear. Try swapping the filters etc. If this doesn't cure it, report the noisy line and see if they will change the gain on the line or locate the fault causing this.