It’s ok to set limits with her as you would with any employee. Imagine if you ran a company. You could set a policy of no personal calls (unless it’s an emergency) except during lunchtime or after hours. If she’s present for a lengthy period of time, hopefully you give her breaks. If the frequency of calls continues, you can point out to her that you notice that she has a lot of “emergencies” and that she’s having difficulty getting the work done. Ask her if she is able to refrain from talking or texting for “x” period of time (2 hours, 3 hours etc). If she says “no”, you have your answer as to whether to continue to employ her.