I've had a few dropped calls (the display on my iPhone froze, so I suspect something on the phone had crashed), I've had it not pick up calls (unlock to answer

I've had a few dropped calls (the display on my iPhone froze, so I suspect something on the phone had crashed), I've had it not pick up calls (unlock to answer the phone -> the phone won't unlock) and calls where I can hear the other party but they can't hear me. iPhone: great internet device, questionable phone.