The only way they might have issues, is if it were skype in a public net connection. Then there would be cameras in many places that would pick up the person accompany in that area. But the details are more finite than you think. Id have to check, but I would say the data packets may very well carry a machine ID or a network ID with them. Mainly they would carry this data for the purposes of the transmission not tracking people. Id have to look up VOIP protocol to see what attributes would be carried with the packet data.
So there is multiple ways of tracking that transmission, something police have access to easily via a range of methods, not excluding the ISP the data packets went through. Skypes pretty amazing as it used to pass through firewalls quite easy.
VOIP has some holes in it and with some trickery such as terrorists accompanied with attractive women using update links etc can enter your machine.
Its not my discipline so I don't know a lot about that aspect of IT. Not that with a bit of research its probably easy to some degree to basic hacking. I understand principles. Stack overloads etc. I just don't bother. But I have no doubt that police can track because, they all start somewhere, and finish somewhere even under an encrypted connection. Police can get hold of proxy servers if they have enough desire.
Its starting to sound like a diatribe of myself. I think we would have William if someone used Skype to contact police or anyone else.
There was a case where someone presented slander in an internet forum of a company impacting their share price. Authorities rang the ISP, got the userID and sued the living daylights out of the perpetrator. That is how easy it is to track you with valid reason.
10 years ago many countries weren't allowed to have SSL encryption. The US relaxed that because their supercomputers can break a dark-net encryption if they want. Not many people have super computers to crack an encryption. Hackers will hunt you either end the encrypted connection. There is only one fibre backbone out of the country
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Even though the packets are encrypted, then sent in to different post offices, they must go through the backbone.