I think the problem is that the BT account pages are very slow to update and, indeed, sometimes never do so. I bought a new iPhone SE at the end of April but the Mobile page in MY BT has not updated and still shows details of previous phone (iPhone 5) and that 4g and Wi-Fi calling not available. This only relates to the old phone and does not affect the connection made by your iPhone SE.
On your new phone go to Settings/Mobile Data and scroll down to 'WiFi Calling' and ensure it is set to ON. Then scroll back up to Mobile Data Options/Voice & Data and ensure 4G and VoLTE is selected.
Once complete go to Settings again and turn off WiFi. You should then see a 4G icon in the top left corner of your screen next to the BT Network info. Remember to turn your WiFi back on again. Hope this helps.
