I often pick up my iphone to find the wifi turned off and the 3 g data on. I'm not sure why this is happening. I don't think there are any issues with my router or wifi. The wifi comes back on in a few seconds, but sometimes I am already at the point of opening an app before this happens. I have a pretty decent data plan, but I would prefer the phone use the available wifi unless and until I tell it otherwise. This is a fairly new phone, which was a replacement for a previous iphone 6 whose wifi eventually totally died after a 3 month long struggle. Any thoughts?