>"It cannot be a "Plain Old Telephone Service" (or analog) issue, because it is working with "VoIP"-- and it's digital since the audio enter at the computer."

Just because is entering digitally on your computer doesn't mean that it is completely digital. To make calls to a land line you still need a PBX in between to make the bridge between analog and digital sides

It's entirely possible that the "crosstalk" happened way before the calls every made it into Skype's domain. Skype will have a PBX somewhere that bridges the gap between Skypes IP protocol and the outside world of POTS and cellular calls.