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called my cell phone, answered, no one there, no noise of any kind, then call canceled.  What is the point of this?

Many robo calls get dropped due to poor connections and by the time you answer, its simply nothing but silence before the hang up reaches you. Hang ups are not always instantaneous at both ends and several seconds of delay is common.

many of the "Silent" calls are scammers looking for working numbers to make lists to sell as "Sales Leads" to telemarketers. the more of these you don't answer, the fewer lists you will end up on.