Depends on what network your phone is on. If your phone is on the same local wifi network as the camera, then no, it is streaming directly over the local network and not using any phone or ISP data. If your phone is on an external wifi network, then the camera is using data on your home ISP to stream outbound, but your phone is not using any cellular data since you’re on wifi. If your phone is on a cellular data connection, then of course it is using data off your cellphone plan.
Each camera uses about 1-2MB per minute in SD video or 4-7MB per minute in HD. That translates to 60-120 MB per hour in SD or 240-420 MB per hour in HD. So 5 hours of viewing would us an average 450MB (or about 1/2 GB) in SD and 1,650MB (1.6 GB) in HD. If you have data caps on your cell plan, then you can see that you can easily blow out those caps with continuous viewing over cellular.