So how do calls between leaders of different countries work? Have all of them agreed upon using the encryption system? Or perhaps they have equipment that can choose which encryption system to use depending on who they are talking to?
If the leaders are important enough, you'll just get a new phone that can talk to theirs. Or a translation server. No need to bother the head-of-state with the technicalities. The point is, there's no reason for there to be a clever, all-in-one solution here, since these are allmost one-off solutions.
That's still a big mystery... but in general, calls between leaders of countries are non-secure. High-level encryption systems are often highly classified, so one country will not easily give such a system to another country. The exceptions are the "hotlines" between a number of countries, where encryption equipment or algorithms are exchanged so a secure connection can be set up.
