Here's how IP trunking makes a phone call happen (without you even knowing you're using advanced technology).
The trunking service run through your service provider is a circuit that connects switches (your system, the receiving party's system) to each other. Because it's using high-volume means of data transmission, such as cable or DSL, it can support multiple calls at the same time (which makes it great for an office).
The same VoIP trunk or network is used to connect multiple users, even remote ones, into the computer/phone framework. Everyone who has a phone in the office has those phones routed digitally through their desktop computers. This is called a privacy branch exchange (PBX). First, you speak into the phone as you normally would. The trunking program on your computer, via a special phone/computer card, then digitizes your voice and compacts it to an Internet protocol (IP) packet, which essentially means that it's "data." The program then addresses and directs that packet to the other PBX -- the person you're talking to on the phone -- via his or her unique computer or IP address. The packet is then sent into the Internet via your service provider's IP network to reach its destination. There, the data is converted back into a voice.
All of this happens instantaneously and alongside packets of Internet/WWW data (e-mails, photos, videos) traveling through the same channels at the same time. This is possible because a phone call is not a "phone call" to the trunking system -- it's data, which takes up very little bandwidth.