My Google skills are off. I can't seem to find out how telephone switches work. How does a 205 area code know how to make it to a 403 area code? I know it's not BGP. So what is it, and what search terms can I use to help me find this information?

What is the "routing protocol" of the PSTN? I (think I) understand it from a historical perspective with switchboard operators and trunks between different exchanges, but I can't seem to figure out how modern telephones know how to dynamically route/switch/build circuits/whatever. A little help please?