If someone calls my iPhone and I answer it through my Mac, can Wireshark collect packets from that transmission?

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If your Mac has a mobile modem in it, it is possible that the calls run through it (so no internet involved), but I doubt this is the case.

If by "mobile modem" you mean a mobile broadband modem, i.e. an adapter that connects to a cellular network for transmitting and receiving data, the Internet probably ''is'' involved - that's the main purpose of those modems, and Skype is an Internet service - but that's a lot harder to sniff from a third-party machine. If you're running a sniffer such as Wireshark ''on the Mac in question'', you can probably capture traffic on the mobile broadband modem, but you'll only capture traffic to and from the Mac, not other cellular traffic.

Attempted to clarify my answer. If you have a better word for the "display-less mobile phone hidden inside a computer", please suggest it.

There are various types of mobile broadband modems that can be built into or plug into a non-smartphone computer, but I think most if not all of them don't support access to the voice part of GSM/cdmaOne or UMTS/CDMA2000 - they presumably support it in the hardware, as the channels for voice go over the same frequency as the channels for data, as far as I know, but they don't export that to the computer into which they're plugged. It's not inconceivable that a built-in broadband modem in a laptop can use the voice channels, just as the built-in broadband modem in a smartphone does, but 1) no Macs have a built-in broadband modem and 2) it's probably unlikely.

@Guy Harris, I positively know at least one model of USB plug-in mobile modem which does support direct (i.e. not over IP) voice calls facilitated by its bundled control application on Windows. But what is important here is your knowledge that such kind of modem is not present in any machine in the Apple product line.

But what is important here is your knowledge that such kind of modem is not present in any machine in the Apple product line.

There are no built-in mobile broadband modems of any sort in any Macs. (There are obviously modems of that sort in iPhones. :-))