It's not. The PSTN isn't one giant cohesive system. It's a federation of lots of companies in lots of countries. Getting such a system in place requires filt

It's not. The PSTN isn't one giant cohesive system. It's a federation of lots of companies in lots of countries. Getting such a system in place requires filtering between carriers as well as everyone filtering what subscribers can do. You may have heard: not every country in the world likes every other country, and not every telephony carrier business likes every other telephony carrier business. It is possible, but easy it definitely is not.