I've had the exact same problem since updating to IOS 10. I was using a iPhone 6 originally, and had hoped the problem would go away once I upgraded to the 6s. It did not. Wifi calling is off, "ask to join"is off, and I have tried resetting network settings as well.
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Here is a screenshot of what I am experiencing. Seemingly, it only happens if a call comes through while data is also being used. It pops up and I have to dismiss it before I can access the call screen. Once I end the call to go back to what I was doing, I am often prompted again and have to dismiss.
I suspect it has something to do with data (LTE) being used while the call is in progress.. I thought Verizon supported data while on a call at this point..
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