I didn't say a baby was more distracting than a phone call, because I think there is a distinction to be made.
To a student who is taking a phone call, it's distracting. To a student who is in pain, that is distracting. To a student who is depressed, that is distracting.
Many things distract individual students, and the thing that enables those students to succeed is a bit of understanding and help.
To a student who is in the room when a phone goes off - no, IMO, it's not wildly distracting. Why would it be?
Most of my students are familiar with the sound of phones. Today, I taught a class where the fire alarm went off by accident. They paused for all of 15 seconds, stood up (as did I), but when it went off and we realized it was an error, the student who was speaking carried on mid-sentance. Why wouldn't he?
I would have absolutely no issue with a student who informed me first stepping out of class to take an important phonecall. I don't see why this is bad?