No, never; however, then again I do not place my iPhone or anything in my back pocket. I also suggest that you do not do that either for, when you sit down, you'll add unnecessary pressure on the phone as well as on your pants' fabric, too. 🙄
Once on my 4s I was talking to someone and when they hung up I found myself talking to my father. Phone did not ring, I did not answer, he just was on the line. Father said he never heard a ring, just that I was on line with him. Only happened once, way weird.
My mom butt dials me all the time. And it usually happens multiple times in a row...usually 3 times and a few minutes apart. It happened with her 5s and now her 6s. She's 71 so I cut her some slack on this issue but I am always surprised how it keeps happened😉. Her phone is set to lock with a password pretty quickly.