It's reasons like this that are precisely why the rules allows students to carry cellphones on them at all rather than ruling them out altogether. Extraordinary/emergency circumstances sometimes warrant their use. For all they knew his mom was in some medevac tent dying.
There's no reason why the teacher, in this situation, couldn't have asked him to take it into the Teacher's Lounge or the Office to have the conversation without disrupting other students or openly breaking the rules.
