How does a phone number actually get routed?
Through a hierarchy of lookups that once corresponded directly to geography and physical switches, and now mostly corresponds to database entries — a change that number portability forced.
The original design. A number was a route. The country code selected a national network, the area code selected a regional exchange, and the remaining digits selected a line on that exchange. Dialling was literally instructing switches which path to take, and the number told you where the phone physically was.
What number portability broke. Once subscribers could keep a number when changing provider, the digits stopped identifying the network. A number that looks like one operator's block may belong to any of them.
So the system changed to a database. Networks consult a portability database that maps each ported number to the operator actually serving it, and route accordingly. Different countries implement this differently — some query for every call, others have the donor network relay it — which is why porting takes hours in some markets and weeks in others.
What actually happens on a modern call:
The originating network analyses the digits to determine the destination country and nominal network.
It checks portability data to find the real serving network.
It selects a route, potentially through several interconnect or transit carriers, each charging the next.
Signalling sets up the call on a separate control channel before any audio flows — which is why there is a pause before ringing.
The terminating network locates the subscriber, which on mobile means querying a subscriber register to find which switch is currently serving that handset, anywhere in the world.
Why it matters practically:
Caller ID is signalling data, not verification — it can be and routinely is forged, which is the mechanism behind spoofed scam calls. Authentication frameworks are being deployed to sign the calling number cryptographically, unevenly by country.
Porting has rules: the number belongs to the range holder and is only assigned to you, outstanding contracts still apply, and initiating the port with the gaining provider is the correct order.
Non-geographic and premium ranges route by tariff rather than place.