Several months ago I installed a UCx 50 switch in a small business that has only four analog lines. The UCx they sent me did not have the onboard CO ports. Instead, they sent me a Grandstream modem, which plugs into the UCx 50. From day one I've had issues with the transmission quality. Specifically, any time someone inside the office is talking on one of the lines, they hear a constant hissing or feedback in the background, but only while they're actually speaking. As soon as they stop speaking, the hissing sound stops. Start speaking again, the hissing starts again. So far nobody has been able to help me, and my customer is close to packing up and returning the whole darn thing. Any suggestions? I'd really like to get this cleared up, because I really like the product, and would like to continue selling it. Thanks!