How Does Making a VoIP call through PSTN Work?

Question

for my college work i was told to draw a diagram showing "information flow" of someone making a VoIP call through PSTN and then explain how it works. After doing research for the last 3h hours stuck im stuck on how a few of the steps work in the process. From my understanding you get a IP Phone, have it connected to your router, router connects to the internet and then from there you are connected to your PSTN provider, the PSTN then sends the digital signals to the receiver(at least from what i understood). What i dont get is a PBX and if it is needed, also how exactly does the Digital signal get from the PSTN all the way to another persons house phone or whatever.