iPhone 8 purchased through AT&T change to T-Mobile I was able to purchase an iPhone 8 through my company with AT&amp;T being the carrier.  I have since left the company and would like to use the phone with a T-Mobile plan.  When I put the T-Mobile SIM card in I receive the message Not Unlocked for that Network.  Any suggestions on how to make this work or can I use the phone w/AT&amp;T only?
Your original provider Controls the network lock status. 


https://www.att.com/deviceunlock/


Procedures differ per other providers, but for you, start at AT&amp;T to check your lock status or request it be changed.