What does "Unknown Caller" mean on cellphone?

Recently i got called by someone twice, i don't know who it is. I was afraid to pick it up on my cellphone because i thought it might be one of those commercial ads. But when i tried to check who called, it only says "Unknown" and "No Number". What does this mean? Is the person who called me... show more Recently i got called by someone twice, i don't know who it is. I was afraid to pick it up on my cellphone because i thought it might be one of those commercial ads. But when i tried to check who called, it only says "Unknown" and "No Number". What does this mean? Is the person who called me from out of my area (Or out of my country and on the other side of the world?) or is it just a commercial ad? Or is it something else? If anyone knows, please answer. I'm just curious of who called me.
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