T-Mobile still allows Sprint systems to limit activation to devices with a whitelist.
Billing systems are not integrated, as it is expected that T-Mobile plans to get rid of the Sprint billing system. There’s employee reports on Reddit that T-Mobile is testing a few changes, that will allow a Sprint customer to stay on Sprint billing and have a T-Mobile SIM, but it’s not guaranteed to go beyond tests.
Some T-Mobile compatible phones, such as the OnePlus 7T that I use on Sprint now, were recently added. To my understanding most unlocked, unbranded devices (besides flagships like iPhone and Pixels) are still locked out.
You would have to check the IMEI or MEID to see if it would work. It currently would also need CDMA, as Sprint UICCs still rely on that fallback.
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