im on the fence for this, tho more so if nanny worked 11/12hr days and no actual break, but if having a break every 45mins then yes ban the phone in working hours
i dont think it hurts to make your child wait for a min or two, and not have every whim jumped to IF nanny didnt have a break
often companies ban mobile phones but they do give out their work landline so people can get hold of them if need be , so sure people at work/offices so have some personal calls made/they ring
evidence 13hr days, wow,im impressed,well done !!
clips the 'nanny' asked for a break every hour 
she works 9-3, you pay her £12ph, is this gross/nett/cih? and has 15mins every hour for a break, so actually works 4.5hrs a day
she doesnt sound like a normal nanny to me (ive been a qualified nanny for 25yrs) and yes i use my phone during working hours
far better to employ her for 4.5hrs and pay her for that, no phone breaks if thats what you want, and she can go out and meet other similar aged children and mums/nannies at m&t, swimming,soft play,parks etc