Well here's a situation I had Ed;

Back about 10 years ago (not knowing any better) I roughed in a commercial unit with quad. The business that was in there had two lines running through the quad and you could always sort of faintly hear the conversation that was taking place on the second line. I noticed this right away and blamed it on the Rat Shack 2 line phones that the customer provided.

Two years ago, a new business ended up in that unit and Bell did the new install for them. I was on site doing the security system for the new owner the same day the Bell techs were there. They started replacing the quad with Cat 3 and when I asked why they were doing it, the tech told me the customer was running 2 line phones and the cross talk was bad. They were now using the NT M9316 phones so I know the phone quality was good. After the quad was replaced, no more x-talk. I vowed to only use the quad in my van for security systems only from that point on!