What is the ruling on a husband saying to his wife: "By divorce, you are the one who hung up the phone," due to his strong assumption and certainty that she was the one who hung up, and then it became clear that she did not?

The preponderant opinion is that your brother's divorce did not occur, and there is nothing upon him, as long as he swore by divorce believing himself to be truthful. Sheikh al-Islam Ibn Taymiyyah said: "The correct view is that whoever swears an oath regarding something he believes to be true, as if he were swearing an oath upon it, then it turns out to be otherwise, no divorce occurs upon him."