keeping data off and using wifi will save you overage .
you can turn on/off but you have to keep an eye on usage.
if you have wifi then use wifi much you can
hopefully that make sense and help you
Turning data off will prevent you from receiving and sending picture, video and group messages. At minimum.
If you have an advanced calling capable device and have it enabled on your phone then it will work over WiFi if your device is WiFi calling capable.
I leave my mobile data on even when I am connected to WiFi. I use 1-2GB per month.
Another question would be is there any reason to turn it off?
Normally, the answer would be no. I NEVER turn my mobile data off. If you are connected to wifi, your phone will use that instead of mobile data if it is turned on. Of course it doesn't hurt your phone to turn mobile data off, but I personally have never seen a need to do so. When I arrive at work, my phone automatically connects to wifi and does not use mobile data. When I arrive at home, my phone automatically connects to wifi and does not use mobile data. I see no reason to add an extra step to my routine if I want to use data while not connected to wifi.
I leave my data off since I mostly use wifi and I send and receive pictures messages, video and group messages all the time no problem. Takes me no time at all to swipe down from the top and turn it on if needed. Mary