Are two devices using my data simultaneously?

Skunkcrossing wrote:

I’m pretty new to this hotspot business, but I’ve learned not to connect my iPhone to my Wi-Fi hotspot and to use my unlimited cellular data instead. I got a FaceTime call, took it on my iPhone, and used up over a gigabyte of cellular data on my phone. Fine. But at the same time my Wi-Fi hotspot showed almost 2 GB of data use. Is my iPad using Hotspot data even though I take the call on the phone?  Do I need to turn off my iPad anytime I’m not using it? Really?

Was your iPhone WiFi setting turned off (and not simply disconnected) when you used FaceTime?

If not, perhaps the data you used was through WiFi rather than cellular data, as WiFi has a higher priority over cellular, and sometimes reconnects to WiFi for data automatically.