I think there's more to the story than what's being said. It seems the customer is having trouble and either Google or T-Mobile should be better about handling the issue but I don't think we're receiving the entire story. I have used T-Mobile exclusively for many years now I have used it when they were being resold by harbor mobile. I'm very happy and satisfied with their service but I have moved both my sim and have placed other Sims from other carriers into my phone and I have never once received a message that my phone has been locked to T-Mobile as a carrier.

Edit: so I had given this some more thought and I don't believe that this is Google or T-Mobile that's put the carrier lock on his phone it's something that Apple allows you to do. It's something that users of iPhone devices can enable or disable in their Apple account. Because I've actually gotten iPhones from family members who are tech savvy who verify that it's an unlocked phone but it would not accept my SIM card saying that it was locked to a carrier. What they needed to do to fix it was log into their Apple account and release the carrier lock that allowed the phone to take a new sim. So this is really Apple anti-theft protection that he's got going on.

T-Mobile was one of the first bring your own device companies where they separated service plans from devices payment plans. I hope he gets his problem fixed and updates his medium post to indicate that this really isn't a Google fi or a T-Mobile issue.