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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!!!

Check the loop current to see if it is running hot. I like to see between 28ma to 35ma but a little lower is ok. When it is too low (18ma or so) you will have disconnects and much higher causes other problems. If you search Loop Current here you will find quite a bit of discussion on line current levels. I see you have Executone listed in your signature line. I once had on old Executone CX that quieted right down after installing 100ohm resistors in place of bridging clips.

MNDAVE When I put a butt set on the line is crystal and I mean crystal clear both on the I-A and Copper line. It seems like if its a issue with loop current I wouldn't have it on both the copper and the I-A.
Randy

Classic case of quantization noise. The mach one is getting old and analog to digital conversion and compression will sometimes have errors which manifests itself as noise, usually a hiss like white noise.

If it is system age, you may just have to drop the DB level a bit so that the customer can't turn it up as loud. Yes, it's a lousy solution but unless someone has another idea, it may still be of some value to check the loop current. Butt sets are much more tolerant than old digital system line cards. If you even try putting a couple of resistors on as a test you only waste a few minutes at worst. It will drop the DB slightly too and who knows, you might get lucky. Remember, the first CD players had a slightly tinny hiss too and the Mach 1 was the first release of this family.