When Finland got number portability some 15 years ago it also got a free look-up service to see the real operator of every number. This was required for price transparency. Calling within your own network can be cheaper than to a competitor and you should be able to know so before making the call.

Not sure how other countries have handled that. The US had the different approach, callee pays for the mobile part.

Have not looked into the issue for a decade...