Same thing, but easier.
Call come in into a switch. IVR answer and collects data. Once Radius aprove the call, you go out on another trunk group to a carrier instead of going out on a VOIP output.
Instead of using a gateway to control the flow of the call, you use a regular switch.
Or you may stay with the gateway and use it to go from one trunk group to another, without using the voice compresion.