If an incoming call is saying it's coming from 555-1234 and telco owns 555-1234 and it's not one of their authorized income points then the call should get dropped.
Or if they don't own it then it's on who's inserting the call into the system.
Telco systems are fun. And by fun I mean it's a complete mess. ISDN is hard and full of caveats (look at SIP for a taste of what it involves).
How? Even in the same city, cellular and land lines aren't always serviced by the same company. In most places I'm pretty sure it's illegal for phone companies to be swapping call data. Or are you suggesting I call back all the numbers that call me to compare bills with the owner on the other end? This plan is not going to work out.
