It's complicated, but basically when you make a phone call it opens a full duplex data connection between you and the tower. This enables both people to be able to talk at the same time. When you do those *228 connections the "talking" done on the other end is the music and it locks your channel down for data connection to download a prl etc. That's the basics of it, but like I said it's highly more complicated than that.
Another one I found out about last week was *22800. Apparently, this forces the phone to connect to the Verizon network. I found this out after a factory reset last week. When the phone restored, I found myself stuck on the US Cellular network.