Ask HN: What happens when you dial a phone number?

I've always been curious about phone routing, but the Wikipedia article on PSTN is rather light. [0]

Back in 2003, I was living in the Midwest. On an August morning, I had a friend visiting from New York, and I couldn't get through to their New York cell phone number. Calls were always busy. Later we suspected it was due to the giant Northeast blackout [1], but never had concrete proof.

So how does cellphone routing work? If you dial a New York cell phone number and the phone is in Los Angeles, does the call need to be routed to an exchange in New York? If a number has been ported from Verizon to Google Fi, does that number still go through a VZW exchange given its prefix? Or is there a DNS for phone numbers that gets updated periodically?

Thanks!

[0] - https://en.wikipedia.org/wiki/Routing_in_the_PSTN

[1] - https://en.wikipedia.org/wiki/Northeast_blackout_of_2003