Best way to end call

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http://forums.androidcentral.com – This is my first post, my first Galaxy s4 is my first smart phone. I'm steadily finding out more about it with Youtube videos but I have what may sound a silly problem, I sometime can't end a call properly! The calls finished, I wan to hang up but quite often the screen is blank and if I'm in bright sunlight it is difficult to see it. Is there anyway I can't be sure that I've actually ended a call. On a number of occasions I've called people and haven't hung up properly, it runs up my bill and blocks the people at the other end from making calls. I must be missing something rather simple (HowTos)