While you can use your phone as a baby monitor in terms of technical feasibility, you probably shouldn’t. Some parents, seeing the rising popularity of WiFi-enabled baby monitors, have wondered if they could simply use an old smartphone as a camera in their child's bedroom and transmit the footage over a video chat app like Skype or Facetime.
While it is feasible to use a smartphone as a baby monitor, it’s not a very practical solution. A baby monitor's ability to see in the dark with infrared night vision is a crucial safety feature, and that's probably beyond the capabilities of your aging cellphone.
Also, video conferencing services like Skype and FaceTime were not designed for continuous use. The call may cut off, forcing you to dash back to the nursery to reconnect the line. The loss of communication during sleep might go unnoticed.