I work at a telco NOC, and while I do work with telephony DMS switches (1U, 10, 100 and 200) I'm not super familiar with the routing but basically land line switches such as DMS100 interface with the phone network using CCS7 and other standard protocols, I'm not super familiar with the specifics but essentially local calls take place within the switch (ex: within a certain group of NNXes which is first 3 numbers of a phone number after area code) while long distance needs a toll switch such as DMS200. There is a central database somewhere (usually within each major telco I believe) that tracks how to route certain numbers for NNX and NPA(area code). I would imagine services like google voice use the same protocols and as far as the PSTN network is concerned it's just another telco/switch. Within the google network itself, then it might be IP based and might use it's own protocol, but as long as the phone network knows how to route it the rest does not matter too much. The google network has to know how to route out as well. I believe this is mostly what CCS7 does. Cells use it too.
For a typical land line you actually have a dedicated copper pair straight to the central office. In the DMS there are line drawers with cards, 1 card per customer. When you get phone service there is actually a physical connection done at the CO to assign your house's cable pair to the card that is programmed with your number I've done this myself though it's not my main job. Typically when a tech needs me to do it if he's up in a pole or what not. So the dial tone you hear actually originates from the central office. In a big city there will be several COs, a lot of them may look like a house and you probably would not even know it's one unless someone told you. In my city we have maybe about 10 of them, some are the size of a house, some are a tiny hut in middle of nowhere. They are fed by the main CO which then connects to the other cities.
Now days a lot of land line service is different in that the dial tone actually comes from a box in your own home. If you have fibre to the home or telephony over cable typically there will be a box that your phones plug into. Though don't think much changes as far as how routing is done.
As far as the actual hardware this is what a DMS100 looks like:
That particular section is the line drawer. There's lot of other perephials that go with it too. Ours has 5ish rows of equipment.
This is old school stuff by today's standards but it's super reliable and still used. Our particular switch was installed in the 70's and has an uptime longer than I've been alive. Everything is super redundant.
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