So we have certain problems with people returning a "missed" phonecall to certain numbers which start with +881. It's said to be expensive.

I've made a bit of research on this matter, and found something. Those are satelite phones. ( , ) This might explain why calling costs are high. A quote from Wikipedia article:

Rates from landlines and mobile phones range from $3 to $14 per minute with Iridium, Thuraya and INMARSAT being some of the most expensive networks to call. The receiver of the call pays nothing, unless he is being called via a special reverse-charge service.

We could also speculate who's interested in such 'frauds'.
Shared publicly