This is what I see under Settings > Cellular, and Cellular Plans and Carrier Services. I don't see an option to transfer to eSIM when scrolling down on any of these screens. "More Plans from T-Mobile" and "T-Mobile My Account" open web pages. "Add Cellular Plan" opens a QR code reader.
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Hmm. Then I guess they only have it for postpaid. That must be fun. I hate carriers. I guess you’ll need to check their app or call them.
Just to make sure, you are on iOS 16, right?