Best not to ring these numbers back! One scam is to dial people randomly from a premium rate number in the hope someone will call back to find out what's going on. Then the callee is charged several quid for listening to a useless recorded recorded message, or is engaged in a time-wasting conversation…
In the good old days phones were hard-wired making it possible to associate numbers to subscribers, and individual calls were metered. Now most of the phone network is packet switched, and phone numbers are ordinary data rather than unique routing information.
Billing is calculated rather than metered. The system is completely different from when my mates did their GPO apprenticeships. For example, International calls used to be expensive, but they're dirt cheap in a packet switched system because a multitude of users all share the same fibre optic, microwave, or satellite links. Connections are many to many rather than one to one, which enables services like Zoom.
As John says, with the right equipment, calling numbers can be changed by the sender to anything he fancies. To trace the call, which could be international, it's necessary to unpick network addresses, which can change over time. All very double edged: not much incentive to phone companies to monitor what goes on for the purpose of stopping crime because they are paid for bandwidth, whilst the same technology makes it easy for governments to keep an eye on communication users. OK when democracies use it to detect terrorists, highly dubious when authoritarian governments use it to crack down on their opponents.
Dave