What do you do if an unknown number appears while calling on your cellphone?

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I have experienced this several times already. There are cases when this happens, I answer the call or won't answer the call at all. The reasons why sometimes I get to answer these calls are first, who knows it's an emergency call. Or maybe it's just a friend of mine who has a new number or maybe who knows it's just a really important call. Sometimes, I don't answer these calls if I'm not on the mood or there is something or someone that I am avoiding and there's a possibility that he or she will use another number just to get hold of me. Do you think anwering this kind of phone call is a must? What do you usually do? Answer them sometimes, put it on your voicemail, or not answer at all?