I purchased my S9 plus on the Samsung website. It is an AT&T phone. I want to unlock my phone and transfer to TMobile. My son is on a plan and wants me to join him. Right now I use the AT&T Prepaid plan. If I can unlock my phone, will I still be able to use the phone before I sign up for TMobile? Will the service still work unlocked if I continue to pay the monthly AT&T plan??