How to unmask blocked calls for iphone?

Okay my bff is getting blocked calls yesterday and she picks up then the caller hangs up. Today they called her again n left a voicemail (in a computrized voice) they said "Hey ***… N other stuff" i'm worried for her is their anyway we can track/ find out who keeps on calling her! Please answer as soon as possible

Added (1). When she didn't answer the phone they voicemailed her. Hello people she's freaking out! Its verbal harrasment… And we have a clue who might be ehind it but we can cause its blocked!

You can't. How about actually not feeling the need like you have to magically answer the phone everytime it rings?

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