To (over)simplify: The worldwide Public Switched Telephone Network, or PSTN, consists of millions of telephone "exchanges", or geographic location "central offices", with phone switches owned by telephone carriers, and all interconnected over dedicated networks. Think of the phone switches as call routers. When one phone number calls another, the switches use routing tables to create a connection between the two endpoints. Traditionally, this was called "circuit switched" telephony, because originally, a physical "circuit" or set of wires, was used to make these connections (now, this is all done digitally, and routed over many different paths via fiber optic, copper wire, satellite and/or microwave). The call connects from one phone, to the local CO, where it is digitized for transit over the network.
Voice over IP (VoIP) calls are "packet switched", meaning: they are digitized right in the device on your premises, such as your OBi box, and then sent over the internet in little chunks (packets), in the same way as all of your other internet communications (email, web browsing, video streaming, etc.).
VoIP calls travel either over the public internet or over a private IP network, eventually reaching the "termination point", where the Internet Telephone Service Provider, or ITSP, then connects them to a telephone switch, as above, where they then become traditional phone calls on the telephone network. The telephone carrier then uses the same phone switches to route the call to your called telephone number.
So: it doesn't matter how the call travels from one end to the other, it's eventually connecting to the PSTN.