If a user has previously blocked private numbers (a common setting to reduce spam) and now needs to receive a call from an anonymous source, they must adjust their device settings.
There are legitimate applications designed to unmask private numbers in real-time. Apps like TrapCall work by exploiting a loophole in toll-free (800) forwarding.
If you previously blocked a specific private number (some phones allow blocking the last unknown caller):
Here’s a solid, actionable guide to on a smartphone.