A dialog box blocks unlocking of my i4 when I get an incoming call.

Sometimes when I get an incoming call I can't swipe to unlock the phone because a dialog box gets in the way of the swipe area. I don't understand the message but it provides three options and I must select one in order to proceed. I click on "Cancel" and then it lets me swipe to unlock the phone. But the call is always gone. The call is not a spam call - it is always from someone in my contact list. Very irritating. Next time, I will carefully make notes of the content of the dialog box, but meanwhile, I'm hoping one of you already knows what it might be and what I can do to prevent it. Thanks.