If your maid using her mobile, you cannot check whether she always talk over the phone. If you want to know whether she is hardworking or not, I suggest you to buy a Panasonic DVD-recoder with Harddisk. It can record the activities for over one hundred hours.
My maid always use my home telephone to talk with friends or relatives whenever she get alone with BB. She also deleted all of her incoming and outgoing calls. But I discovered that the phone became very dirty everytime when I came home from work! She wipe the telephone everynight before go to bed.