Anywhere in the world
Good luck with that.
You may be able to get around SIP's default hostility to NAT and the distant (possibly mobile or totalitarian state) network's hostility to data that isn't HTTP if you go to the trouble of establishing a VPN (assuming said distant network permits), at which point you'll run afoul of the traffic shaping and network latency that turns any attempt at conversation into an exchange of whalesong.
Meanwhile, the "proprietary plastic thing" will achieve the basic necessities of communication with little intervention - apart from dialling the number and footing the possibly excessive bill.
VoIP can work well in a carefully-controlled environment. On the "anywhere in whe world" scale, though, it's a poor trade of cost against quality and convenience.
Biting the hand that feeds IT
