does the nanny regularly have her phone switched off or was it an accident?
yesturday i switched my phone to silent at singing at 9.30 them forgot and turned it back to ringing at 1ish... i checked for calls / texts (and hadnt recieved any) but if i had i would not expect a written warning for missing a call....
on the other side (almost) there was a time when i REALLY REALLY needed MB or DB... their dog became extreamly sick (and the vet said it nearly died, with just hours to spare)... anyway i took control and called and texted both bosses AND called their work numbers, and called their work headoficces to try and locate them...
i couldnt reach either of them and was responsible for the poor dog and vets etc all afternoon. The point was it COULD have been one of the children seriously poorly, and i could NOT get hold of either of them at all...
anyway, rather than wating for them to come and sort the dog out i sorted the poor dog myself (which from your analogy would probably equate to you collecting your child who needed collecting)
ofcourse while i am working i make every effort to have my phone on and with me... BUT some places it has no signal, some places its on silent, and occasionaky the battery runs low (I keep a charger at work too and this is rare but it happens cause its LIFE).... and yes twice i have forgotten it and done the 90min school run without my phone... because i am HUMAN
I also have the names / addresses / numbers for all my nanny friends, and 'our groups' etc in the nanny diary so IF my phone didnt work and it was that important, my bosses could use the nanny diary to see where we are and find another number etc.
i think a friendly conversation is the way forward, along the lines of can you try to have your phone n while working... and maybe suggest writing the names and numbers of her nanny friends who she sees, so that you have alternative numbers too would be a way forward
also have you ever accidently not had your phone switched on, or on silent etc?