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How can I only block private numbers on a cell phone?

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Well its not my phone but its lgu300 and well like I've been gettin prank calls on it when it is my mums phone.I could get in trouble if she finds out people ringing on it. I dont know how they got my number. Well I went on settings and blocked people who I ahvent added in contact list. How can I only block private numbers?? Theres no option on the phone sayin block private calls.