I want calls coming in one trunk (zap) out another trunk (sip) transparently to the caller. One way that comes to mind is to modify the inbound route for zap and connect the ‘set destination’ to a custom destination as follows:
You should be able to do this in the GUI directly. Create an inbound route to receive call calls you want to forward and choose a destination of Trunk, then select the appropriate trunk from the list.
Yes, I agree with @lgaetz. It can be done through the inbound trunks, but it will be dialled as is through the trunk. Sometimes, there is a need to change or to add parameters to the Dial application, and that is why I prefer to use my own contexts.