Amplified background noise


Can anyone suggest anything to help with the following small thing.

I have asterisk 1.2.11 setup with a Digium 205p card on our bt isdn30 line.
In zaptel im using the mark2 echo thing with aggressive echo cancel enabled.
TX and RX set to zero.

On our local network we have some grandstream gxp2000 phones which are now working well.

Some of the calls we make are to busy shops and offices and sometimes its like the background noise in these shops and offices ie people chatting,clattering,walking etc is amplified to much which makes it hard to listen to the person on the phone with all this noise.

I know we dont have this problem on a old norstar phn system asterisk is replacing.

Has anyone any ideas on what I can try.