Why won't my phone allow me to answer certain calls?

My Alcatel a30 Fierce is a piece of junk. It often changes setting while it's in my pocket with the screen locked. Two times it blocked numbers without my knowledge. I unblocked the calls but on one of those numbers it still will not let me answer when that number calls. It doesn't give me a ringtone. The banner appears saying to either dismiss or accept the call, but the banner always goes away before I can touch accept. Even when I'm quick enough to hit accept it still does not. All settings appear normal.