>I can't see how they'll manage to argue these aren't calls in a court //

I'd have thought on the basis that a telephone call involves two parties in simultaneous communication, the caller and callee, and that a voicemail box isn't a party when a message is being left; the recipient collecting the mail does so of their own volition; the communication is not a phone call, it's asynchronous.

Whether that will wash probably depends on caselaw that I have no knowledge of.

Even with the carrier/country specific techniques everyone is mentioning, they all involve calling, so they should fall under existing law too.