I am ordering the iPhone 8 Plus on the iPhone upgrade program andI had to enter my AT&T carrier info when ordering. I understand I'll get the GSM version with Intel modem, but the phone is supposed to be unlocked. My question is do I have to activate the phone on AT&T or can I just activate it on my other GSM carrier? Or do I have to activate on AT&T first then switch sims? Does activating on AT&T require me to subsequently contact AT&T to unlock the phone or does it stay unlocked? Basically I just want to receive the phone and activate it on my other GSM carrier without going through AT&T.