I'm aware that some countries do their best to block VoIP services, or just about any service that by-passes their incumbent carrier, but people still manage to find ways around it, usually with VPN. I have not heard of any legal implications of using VoIP services from one country to the next, if you can get it to work. I think it would be a different matter if you were attempting to sell telecomunication services in a country that deliberately blocked or forbid such services.
If you have been warned by authorities NOT to use VoIP services over Wifi/internet in a country, and you choose to ignore, then you take a risk of a knock (or more) on your door. But, that is the risk taken of the person in that country, I cannot see how they could do anything to a company operating outside that country. Many large corporations have VPN, for their employees to use while travelling. If they can get it to work, then the county/ISP that doesn't want you using it, isn't doing their job if you can get through. Just look at what happened with China and VPN services a while back.