Many years ago I worked for a VOIP provider. The most expensive frauds that we detected was calling special service in remote place of the world like Tristan de Cunha, Nauru, Falkland or Buthan. Something like 50$ for minutes and 10$ at the response.

Aside from fraud and weird toll numbers, for bonafide use its often calls (esp to mobile) in certain countries where there's less competition, often smaller island nations like British Virgin Islands or the Maldives https://www.voipkredi.com/page.php?page=betamax-dellmont

For a while there was a lot of in-cahoots fraud where a country’s telecom would make seemingly arbitrary numbers a premium rate $$$/min number and some uncareful VoIP providers got hosed.

https://www.forbes.com/forbes/2005/0919/058.html?sh=3e7f98cc...

Not a destination but my guess is one of these would be the most expensive call (I think some people here are young enough to not be aware of these): https://en.m.wikipedia.org/wiki/Premium-rate_telephone_numbe...

a few small pacific island nation-states that have no submarine cables and are entirely dependent upon geostationary satellite capacity and o3b right now, most likely. this is still satellite but does not go to the "special" country codes for calls to iridium or inmarsat.

there's also a few "special" toll-bait destinations in the caribbean which are unusually costly due to the fees charged by their local telco as a revenue source.

North Korea I believe.. then all the little Pacific islands.