No, how would my uncle in the countryside of Vietnam do that? He doesn't have a credit card -- not many here do. He doesn't speak English -- can you find a website that sells voip numbers in Vietnamese? Buying a voip number from a provider in Vietnam has the same exact KYC requirements as buying a SIM, so it is still tied to your government ID and registered forever.
Also buying a VOIP for 1 month costs something like $10 from a quick Google. Average salaries are like $1.50/hour. Nobody is going to pay an entire day's salary to buy an VOIP number they throw for a month just so they can register anonymously for chat.
So, not you can't "just" buy a voip number unless you're a rich Westerner. But who needs privacy more? People in liberal democracies or people in places like Vietnam (literally an authoritarian country where people are routinely imprisoned for speaking against the government)?
> I don't know anyone who buys a phone with cash except international students.
Everyone buys a phone with cash here because few people have credit cards, since there is no such thing as "credit ratings" and it is easy for people to disappear from their debts. There are more people in Vietnam than any country in Europe. We all use smartphones and messenger apps here, too.