is this a loud chirp or squeal sorta thing?when you say tone i think touch tone type.
possible loud chirp, clik clik, squeal.?
what kind of trk is comming into this pu grp.
and is this trk on a t-1 or copper?
if it's copper, you may want to switch cards of the incoming line. then the analog card, way back when, certain vintages of caards were better than others for noise and other things too! or maybe the line comming in is too! hot!
to high of db level from card, or telco. good luck