Since you have interference at the Dmark prior to being fed into the system your probable solution is the filters previously recommended. Your ground is probably OK. I would place a call to the service provider and alert them to the problem, after all it is their responsibility to deliver you a "clean" line to work with. Who knows they might dispatch a tech with filters to install for you. Definitely bring it to someones attention such as customer contact. You are repairing someone else's problem which brings up another question. Did this customer choose a new phone system because of this particular problem or were there other factors?

John