I know the best policy is not to pick up an unknown number and to let it go to voicemail, but there's also this option - Any one of the following -

Paid but cheapest -
https://itunes.apple.com/us/app/robofence-block-robocalls-respects-your-privacy/id1148180468

Free/IAP -

https://hiya.com/

https://www.truecaller.com/download

https://itunes.apple.com/us/app/nomorobo-robocall-blocking/id1134727588

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