I bought the s21 Ultra through tmobile and used a new sim card they gave me but the phone came with a sim card. The issue that I'm having is my phone is on 5G all the time it literally never drops 5G until I make a phone call. Soon as I make a phone call it drops down to 4GLTE and my mobile voice network type drops from NR SA which I believe is (New Radio Stand Alone 5G if I'm not mistaken. My phone is always on 5G, 100% of the time until I make any call and my mobile voice network type is always on NR SA as well right up until I make any phone call. Could this be SIM card related perhaps? I still have the SIM that came new in the box with the s21 Ultra. Only reason they put a different SIM in my phone is because I had just gotten a new tmobile SIM card a week prior. They both say R15 on them, but the design on them are slightly different. How can I tell if this is affecting my VOLTE? I'm considering going back up to a tmobile store to register the SIM that came with the phone but I don't wanna waste my time if it's not the issue. They take hours in these stores to do anything and they try to charge me for every little thing.