I always buy my phones directly from Samsung so they are unlocked. I traded in my S20 Ultra for an S21 Ultra with Samsung. I kept the AT&T Sim card and put it in my new S21. I had to call AT&T to let them know what my new IMEI number is and they do something behind the scenes and then that's it. Well I was trying to get the recent Beta update and people were saying that AT&T wasn't allowing their customers to get it because their phones are locked. Others were saying that if you have an unlocked phone bought directly from Samsung you should be able to get it. So this got me thinking, if someone has an unlocked phone and puts an AT&T Sim into it can AT&T then lock the phone?