Earlier quoted context omitted.
> you can originate a call on the internet that's nearly impossible to trace. // Why do the phone networks route calls with faked originator numbers. I can see why they don't block genuine numbers (that terminate at an actual phone) -- to get you to pay for blocking. But surely they can tell that an incoming call from a foreign call centre with a local number is fake. Recently I've had calls, in the UK, from numbers…
Phone numbers are not necessarily symmetric. A given number might legitimately place a call via someone else's network. For instance, think about roaming. You're calling with your local number, via another country's network. So, it's not a super simple problem to solve everywhere.
Now, when you call me, the HS gets "the call". It then contacts the VS. The VS will assign a temporary phone number to my cell, and send this number back to the HS. The HS then sends this number back to the switch making the call. That switch will then call the temporary number, hitting the VS. The VS sees that number and since it assigned that number to my cell, knows to "ring" my cell phone.
And that's just for cell phones. That doesn't cover call forwarding, for instance.
[1] Number portability adds another layer of indirection to this.