Any kids on helper's hand?? If yes, are they old enough to protect themselves? If yes, tell the helper you will fire her if she continue to use phone during working hours. Otherwise, hire another maid to replace this one la.
i have informed my maid that using mobile phone is not allowed during working hours or with my baby, but finally she still used it twice (but I found it from web cam) after giving her verbal warning, therefore, i kept her phone and return to her only on her holidays.