hi, I recently got my dad an a series for reason of wifi calling and also had issues. now I am not sure what your network operator is, but ours is EE UK and the tech support suggested the following which does work. go to settings, connections, mobile network the toggle VoLte calls sim 1 to off. this will make sure your phone prioritises wifi calling over 4g mobile network. now might not be issue your having as like say not sure what network your on or if its 4g or 3g, but if hope helps.
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