I cannot tell you how how inappropriate it is for a nanny (or housekeeper) to be talking and ESPECIALLY text messaging on their cell phone while at work. The ONLY exception would be in the case of a family emergency. In the case of an emergency, as soon as you walk in the door of your Employers home you should LET YOUR BOSS KNOW you may need to be on your cell phone due to an emergency. Otherwise PUT YOUR CELL PHONE AWAY (on silent) or TURN IT OFF while at work.
I won't even get started about being on your cell phone while at the park- EVERY nanny should know better!