How do I block my grandma from calling me?

My grandma is a widow and has too much time on her hands. so she calls me like every week to check up on me! it's so annoying. why can my grandma be cool and just buy me beer or something. anyways I need to block her because I don't want my friends thinking I'm a loser grandma's boy.
How do I block my grandma from calling me?
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