When I have both my WiFi and mobile data on, I have found that it only uses the WiFi and not the mobile data.
I know this because the modem was turned off in the house and the internet was down.
I could not load a web page on my phone with both WiFi and mobile data being turned on.
After turning off the WiFi on my phone, I was able to load the web page because my mobile data began working.
I turned the WiFi back on and could no longer load web pages.
That was enough proof for me that while WiFi and mobile data are both on at the same time,
the phone is only using WiFi.
The reason I am posting this here is because I could never get a straight answer from anyone.
People were not answering the question asked and were only suggesting that I turn off my mobile data when my WiFi was on.
I have also noticed that when WiFi is on that my apps will update (Because I have it set that way in settings)
When WiFi is off I am only notified that an update is available, so I know that these updates are not using my mobile data.
All of this of course is when I am within reach of, and connected to my router.
If I walk out of my house, get in my car and drive down the street, out of range,
although my WiFi is still on, the phone will then begin using mobile data again.
I have not yet tested to see if the phone will update apps using mobile data while WiFi is on and out of range...
I will leave this for another day and another post, unless someone can answer,
and give some definitive proof, as I have just done.
No half-cocked answers please.
If you have both WiFi enabled at your home or WiFi hot spot and have cellular data on there is always a possibility that you will get dinged by using cellular data. WiFi can drop or you can have a spotty connection or interference etc.
The suggestion to turn off cellular data and data roaming insures the phone does not hit your cellular data allowance.
Hope that was straight enough?
If your phone is connected to a WiFi hotspot/router, the phone will default to only use that regardless of whether on not the hotspot/router itself has an internet connection.
This can/will result in webpages not loading due to the router not having an internet connection. The phone typically will not automatically default over into the cellular signal if the router/hotspot doesn't have an internet connection. The phone "assumes" that you want to use the WiFi rather than cellular.
You would need to manually disconnect the WiFi at that point (or turn the router/hotspot itself off so that it is not broadcasting).
Ok then to recap...
There are 4 states of WiFi on a smart phone.
1) Wifi on but NOT connected
2) Wifi on and connected but No internet
3) Wifi on and connected with internet up and working
4) Wifi off.
If mobile data is turned on or enabled, using the above examples...
1) phone uses mobile data
2) phone uses Wifi
3) phone uses Wifi
4) phone uses mobile data
Quite frankly, this is how the manufacturer programmed the phone, NOT VERIZON! It may be a bit unnerving but you have to learn to turn off the wifi when you want cellular. Otherwise, they are going to automatically connect to your wifi in an attempt to save their network and YOU money.