Unmasking no caller id calls- iPhone. Help?

I have a friend at my work place that is being harassed with countless phone calls. We are aware its by someone from work as he has been called in by said person for a "meeting" at work that obviously didn't exist.

I've heard of websites that allow you to unmask the numbers so instead of it saying "no caller id" the number will come up and we can report this to the manager/ police. However, they're all subscriptions and would prefer something thats free.

Would the phone provider be able to give my friend the number thats ringing him or is there another way to find out who this person is?

I guess another option might be to block anonymous calls on the iPhone but would prefer to know the number.

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