I'm not sure what you're getting at - if you have both a Wi-Fi and Mobile data connection, and the phone needs to use the Mobile data connection, it will: ex. MMS requires a Mobile data connection, so even if you're surfing the web using the Wi-Fi connection, if you get an MMS you'll briefly see the Mobile data icon (LTE, 3G, whatever) in the Status bar along with the Wi-Fi icon, and when it's done transferring the message the Mobile data icon will go away.
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