It's 2020. Pretty much all of the PSTN is digital. So, when someone places a VoIP call through an ITSP, where do the bits actually go?
For example, when you place a call to a PSTN number through Callcentric, Callcentric first decides whether to handle the call internally (e.g., to another Callcentric user), pass it upstream to "the PSTN", or do something else. When Callcentric passes "it" "upstream", what exactly happens technically? And who (between the telco companies) pays for it?
Given that everything appears to be hell-bent on sticking with G.711u, can one reasonably infer in 2020 that the call stays unchanged as G.711u bits for the entire journey for non-cellular network phone numbers, and for cellular network phone numbers, that there may be one transcoding between G.711u and the mobile operator's preferred codec for the associated mobile device?
I am asking specific to North America, including inside the NANP. No need to discuss international toll calling.