The only time you would use Wi-Fi for voice calls would be when making a VOIP call via an app such as Skype or Viber. The only thing I can suggest is to disable Wi-Fi before you make a call by dragging the notification panel down and turning Wi-Fi off from there. In fact, you can turn your Wi-Fi on only when you need to send/receive emails or surf the interweb and turn it off when you're done.
Ray.
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