According to a statement released by the carrier, they are blaming it on a bug iOS 15.2 and that they have shared this issue with Apple. “Overnight our team identified that in the 15.2 iOS release, some device settings default to the feature being toggled off. We have shared this with Apple. This is not specific to T-Mobile. Again though, we have not broadly blocked iCloud Phone Relay.”
That being said, there was some reason to believe T-Mobile could be blocking the feature. This is because there have been some carriers in Europe that have indeed blocked Apple’s iCloud Private Relay, so it stands to reason that it is entirely possible that T-Mobile could have done the same, but this statement suggests otherwise.
However, the carrier does note that in some instances, the feature is blocked. According to T-Mobile, “Customers who chose plans and features with content filtering (e.g. parent controls) do not have access to the iCloud Private Relay to allow these services to work as designed. All other customers have no restrictions.” This means that unless you have those content filters enabled, by right it should not be blocked so hopefully this is something Apple will address soon.