Using a T-Mobile Locked phone on T-Mobile MVNO?

I would love to hear some real world experience here - has anyone had luck using a T-Mobile locked phone on an MVNO? I’m getting mixed messages all over, and hints that a recent change might have taken place.

Would love to hear your real world experience! Trying to find my brother a cheap phone for his Tello plan, and unlocked phones are at least 50% more expensive on the used market.