call was just silent...has called our number numerous times

Call was silent

Just received a call from 3215748690. I said 'hello' and there was nothing. I waited a few seconds and then a woman said 'hello'. I said 'hello' again and she asked to speak to my son by name. I said he wasn't in (he's in high school) and could I take a message. She said there was no message she would call back at another time. Don't know where she was calling from or why.

Open Line, no one talked  ????