I have a customer with Mach 1. We changed the copper lines to I-A (this has nothing to do with the issue) when we made the change the customer got on the lines and said he could hear static. I went out and tested and could hear what I would call a hissing sound when the volume is above 4 # signs on the display. the more #'s or higher the volume the worse it got. Thinking it was related to the new lines I tested at D-mark and all is crystal clear. Still thinking maybe some type of I-A issue I took a dedicated copper line off the alarm system and tied it down on line 6 in place of one of the I-A lines the sound did not change on the handsets (all phones are the same 3 cards in system and all the same on all cards) I'm confident the sound has always been there but because of the new service the customer was listening closer than he ever had before. I tried wrapping a magnet around the lines as they went into the system and had no success, tried DSL filer just before lines go in and no change. I'm sure it's inside the system somewhere. I also played withe the gain the book says not to adjust without tech support telling you to but that only lowered what the caller heard...... any help would be appreciated!!!