What is the dial prefix function of the Voip?

The dial prefix's are defined as "remove prefix" and "add prefix" and they are used to insert or strip numbers before the dialing digits.
The inbound phone book must use the "remove prefix" to match at least the leading digit dialed into the voip from the IP network.
For example, if channel 1 is FXO and connect to a PBX analog station port, and your PBX extension range is 1000.
You might create an inbound entry with "remove prefix=1 and add prefix=1, channel = 1"
Now when 1000 through 1999 are dialed into the voip, the voip will send 1000 through 1999 to the pbx, hence ringing that extension directly.