YABU, for all you know it might be the only time of the day when she could actually have a break and considering you said she did notice when the child started crying and attended to him/her, it leaves me to think she wasn't as engrossed by her phone as you suggest she was.
As someone who has nannied full-time, you might have seen me with my phone while out and about with the kids every once in a while (though not always) and probably would have judged me for it but the truth of matter was that I was also the one who would be up at 6am to look after the baby/kids while the parents slept in until 11am or 12pm everyday, and then spent the entire day with them until it was time for them to go to bed, I was also the one who would stay with them for up to a week while parents went away on holidays without them, leaving me to care for their little one 24/7 with zero break and the one who had to fit in cooking, cleaning and doing their laundry during nap time (my only breaks) because doing it when they were awake would mean not being able to pay full-attention to them, so yes sometimes I did use my phone when out and about when the kids were busy playing with other kids in an enclosed environment and didn't need hand-on care, I would still see them and intervene but no I would NOT follow them around if I didn't need to and shock-horror but lots of the parents I did work for who may have spent a grand total of 2 hours with their kids during the day still managed to check their phone during that time frame or even just hand it over to their kids to entertain them (instead of entertaining them themselves). Would you call SS on them? (Or you judgement is reserved for nannies only?)
So judge away, but reporting a nanny who responded to a kid's need when needed because she was texting when you have no clue what her day looks like is in my opinion ridiculous.
I personally don't understand parents who have higher standards of care for their kids by a childcarer than they have for themselves as the parents anyway (you admittedly checked your phone yourself yet would be horrified if your nanny did?).
Unless you actually fear for the kids welfare I think you would be incredibly unreasonable to report.
And I used to be a teacher, where I had DEDICATED break hours where I could check my phone, nannies don't have that and apparently shouldn't ever check their phone while working without being reported, never mind that they have a family of their own and might have things that need attending (without it compromising the safety of the kids).