I have a hum in a 2 line phone system. I recently switched one line over to VOIP ATT Uverse. So I have a VOIP line, and a regular POTS line. I haven't had this problem before on the two phones that I have:
I have tried reversing the tip and ring on the POTS line with no success. When I plug in one line at a time, the hum goes away (the problem only happens when both lines are plugged in together). The hum is noticeable on both lines. I also tried converting the line1 and line2 into 1 4 pair line. This didn't help. I read somewhere how this could be a ground loop problem. I also tried buying a new ATT 2 line phone from Radio Shack...that didn't fix the problem either.
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I still need your help. I signed up for the $15 a month subscription. I am actually at the location right now working on the phone lines. I tried adding filters to the Uverse line. That didn't work.
From what I can tell, there are 8 or so 2 line (separate 2 pair) jacks around the area. When I connect at the test jack, the phone line is fine. When I connect at the wall jack (one line), the phone is fine. But the problem only happens when I connect the 2 line phone to both lines. I can unplug one of the lines (either one) in the 2 line phone and the hum goes away. This is making me believe that it is something to do with a ground loop in the phone possibly. I have 2 of these phones and have also tried a new one so I know the phone isn't bad, maybe just a digital/POTs limitation. I would love to know a fix for these phones. I tried a different A/C adapter, switching the tip/ring on the POTS line...neither of these worked.
You definitely have a ground cross talk issue going on here. A filter that can ground the noise to ground usually works in theory but isn't always the case. I can tell you that directing digital and analog to a single device is never a good idea because they dont share the same internal ground circuitry and you'll always have some type of crosstalk. As long as the crosstalk uses the same internal ground...you ok, but that isn't the case here. I would switch both to VOIP to be completely honest. You can try adding a filter capacitor between the two lines and see if they cancel out the hum, but it's only a suggestion. Hope this helps my friend and please remember to only leave a rating of OK Service or higher.