Please note that these steps will prevent your device from connecting over LTE.
1. Go to Settings > Cellular Data Options > Voice
2. Select 3G or 2G, and that’s it!
Only do this if you need to prioritize receiving important calls or SMS, or if you are in a location with poor LTE connection.
This trick will not work if you are in an area with no cellular coverage at all.
Just re-select LTE if you need to connect to it for browsing.