@BrettJarvis Thank you thank you thank you for the direction to Samsung Chat. Very helpful and resolved the issue in no time!
I have been having exactly this issue between Virgin Mobile and Public Mobile. I have been back and forth for 2 days trying to figure this out. In the end, this thread was a life saver. I tried Virgin Mobile, but they insisted they could not help me find the master number. I cut that convo short and then contacted Samsung directly by chat. The probkem was fixed in 5 minutes. Thanks for posting this!
I have a Samsung Phone and it was asking for the Master Unlock Code. I chatted with Samsung online and within minutes they gave me the Master Code. After adding that, I was then able to put in the Unlock code from my old carrier (Bell).