Well, I have a couple of ideas worth looking into:
#1: You should be measuring your line voltages using a DC scale. 45-52 VDC across an open line is normal, ring to ground should be about the same and tip to ground should be minimal. In a perfect world, the tip-ground measurement should be as close to zero as possible. On POTS lines, the tip side is grounded at the CO, so any tip-ground voltages measured will be only minor induced AC influence.
#2: The PC that you are using for your Magic Jack must be grounded. I know, I know, most are. Bear in mind that if it's not, then the voltage from the Magic Jack is "floating", as in no reference potential to ground. I'm no computer whiz, but I'd like to think that all power supply outputs in a computer have a reference point to chassis (earth) ground.
A difference in potential to ground between the two "CO lines" that you are using may be the source of your problem. The common internal ringer circuitry in the telephone set relies upon basic, and VERY basic DC isolation since it's fairly common for both CO lines to have the same reference to ground. This is due to the fact that they typically originate within the same central office and ride over roughly the same length of cable. With two lines that don't originate in this manner, that could be the cause for your issue.
If all else fails, try isolating the computer's chassis ground using a 2/3 prong adapter to isolate the chassis ground by not connecting the grounding terminal on a temporary basis. This will at least help isolate the potential AC interference due to the local ground. Since the Magic Jack is powered by the USB port, there's a good chance that may very well be a floating voltage, causing excessive AC influence between the CO and the computer's power supplies not being bonded together.
Bear in mind that the Magic Jack was not intended to be used in any kind of sophisticated environment. The manufacturer's intention wasn't really much more than to connect a stand alone single-line set to it.
Typical two-line phones aren't sophisticated enough by design to take these issues into account.
