Hi - So I’m a bit confused as to how you want this to work based on your description but I’ll take a shot at providing a solution.
I think what you want is an “Error Boundary Event” on the call activity.
Master model:
This way, the subprocess can either complete successfully and your master process can continue, or it can throw an error and end the master workflow.
Subprocess model:
You are correct in that, as is currently implemented, a “terminate end event” is functionally no different than a normal “end event” so that is why when the subprocess completes, the master process continues to the next step.