It's opt in. In a radio segment the other day they said 6k people have volunteered. Which makes it very much non-random, but still, cute concept.

probably a good thing, you wouldn't want most of the calls to be answered by people very unhappy to be cold-called by some random foreigner...

The fact that they are volunteers doesn't change the fact that it's random, it just changes the set from which the random values are selected.

You have to actually opt in, as an ambassador.

From what I can gather from the FAQ, it's entirely opt-in. Swedes sign up to be cultural ambassadors.