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Yet if I have my MagicJack Plus plugged into the USB port of the PC the Line In Use immediately goes away. I also noticed this appears to be an issue with the phone more than the Magic Jack because if I unplug the phone cable, the phone will say Line In Use no matter what even when nothing is plugged into it.
From what I read this seems like a voltage detection issue and perhaps the phone got too much voltage, or damaged, it is just weird how MagicJack Plus only when in USB will work but not when A/C outlet, tried 4 different AC outlets and cables. It seems to send more voltage to the phone when it is plugged into USB or something.
Because the phone does it no matter what even when nothing is plugged into it, I simply plug in MagicJack while it is in USB and the Line in Use goes away, then I unplug phone cord from phone, and Line In Use comes back. Replaced batteries, as some said if battery is low it could cause issue and no luck there.
I likely will end up buying a replacement phone, but this tells me that the Line In Use problem may not always be a MagicJack issue but can also end up being a phone issue as well.