MacB, I agree that it is about focus. If the child/children are occupied doing something - they may be entertaining themselves, not needing lots of interaction - then the nanny does not need to focus on the child/children and can take a mini-break to send a text.
I use my mobile quite a bit during my working day, generally early on it's texting Childminder friends to see if we are meeting up anywhere, such as going for a walk, to soft-play, swimming etc. Then I will often use it to look up recipes, places to go and I can use it in-car as a SatNav. I try to avoid answering calls from unknown people, I actually avoid all calls as much as possible unless callerID shows it's from my boss.
Phones these days are multi-function devices... mine is my watch, a camera, SatNav, internet browser (for looking things up, especially when 5 year old asks what the tallest building is in the world and how many lifts it has!), GeoCache finder (reminds me, must take the children for another treasure hunt, they like finding the treasure boxes), walking tracker (it can track our walking routes and show how far we went, what speed etc - I have some OS walking Maps on it, so it's a bit like a SatNav for walkers), it has instruction manuals on it - so I can make a loaf of bread using the bread machine, what else does it do... hmm, oh it can check my e-mail and thus also Twitter Direct Messages, sends texts and can make/receive phone calls (don't get those often though).
Texting is a small part of what a phone will do these days, so if you see someone using a phone, they may not be sending a text... they may be looking things up on the net which could be work related - a recipe, bus/train timetable, checking the weather forecast etc.
If children are being ignored to a large extent then that is wrong. It's about focus and getting things in balance. If the child needs the nanny, then the child's needs come first. At other times, the child may benefit from not having 100% of the nannies attention. Children need time to explore for themselves, they don't need constant attention... just a watchful eye. Pity my eyes don't see through walls - as it's when I'm not in the same room that I find issues can occur!