Opening up the phone was for the parents’ benefit, not the kid’s. They took the path of least resistance because they wanted a reduction in volume/anxiety. Personally I would have either let the toddler cry it out and possibly take a nap, or figure out what was wrong. But I suspect the parent knew what was up and just wanted to end the tantrum. Not exactly setting that kid up to be able to cope with things later in life…
-Tom