What was the purpose of the cell phones Richard gave to both Vanessa and Emily in the novel, The Wife Between Us?

The cell phones that Richard gives to both Vanessa and Emma symbolize his possessiveness. He has the cell phones programmed so that he can check the location of the phone, thus keeping up with where these women went during the day. Vanessa learned Richard had a location finder on her phone and knew to leave it behind when she went on errands she did not want him to know about. She warned Emma that Richard could track her through the phone he had given her.