Thanks @StuF.  It just started happening recently and only occasionally.  I have tapped, swiped and growled.  Nothing would open the call.  So frustrating!  I have all the updates.  I borrowed a book from the library but took it back before coming to grips.  I thought it suggested the call might be a facecall and that I should swipe the green icon right.  I'll call from a phone box as soon as I think about it and see if I can home in on the problem.  Very happy with the phone otherwise.  Perhaps I will have to find how to contact Oppo otherwise.  Thanks!