If you are uncertain, send all calls to VM, and whitelist the one you want to get through. Once you know they can get through, then switch the rest to "disconnected".

I expect whitelist always take priority.
I also found that most calls routed to VM do not result in messages being left.

I send all unknown calls to VM, then allow certain numbers and local area codes to ring through. No toll-free numbers ring my phone. If something is important, I expect a message to be left. At least that works well for me.