Who I have the same questioned this topic

I have three iPhone 5s. All are using my WiFi while at home. One iPhone is sending data over Verizon's network, not WiFi, every 6 hours or so. The time of this data transfer changes every few days. I see data usage number on my Verizon account ranging from a few kilobytes to almost a gigabyte. Is this due to Verizon polling the iPhone for data usage and billing purposes on a regular basis? And what do these billing numbers mean? Where does Verizon get these numbers? Any running apps while on my WiFi should use the WiFi, not Verizon, correct?