This is how Corona works: Mr A is travelling overseas. Mr B...

This is how Corona works:

Mr A is travelling overseas. Mr B calls Mr. A. The call is put via PSTN (Public Switched Telephone Network). PSTN refers to international telephone system based on copper wires carrying analog data. This goes through Corona which routes the call using: local sim tech, wifi or lte.

PSTN is a very old system and cheap too. That is why Corona uses it in long haul. Also, the use of local sim, wifi or lte is short haul and cheaper as well.

It doesn't have to go through cellular network or use a voip service. Therefore, instead of just saying 'hello hello' on voip, you can actually conduct some business.