Caller ID said 'NOT IN USE'. I almost laughed. Someone chose that one. So, since it is not in use, I picked up, but said nothing, and was responded to in the same way. After about 20 seconds I just hung up.
This and other calls like this I don't know they called till I look at my phone ,cause unless they are listed in my contact list i have a silence ringtone and never hear the call and just delete before I turn off the phone