The T-Mo Report (who knew there was a site dedicated to T-Mobile news?):
According to internal documents shared with us here at The T-Mo
Report, the blocking of Apple’s privacy-focused service is
actually due to a conflict with existing content filtering
services on T-Mobile. [...]
This seems to indicate that the blocking isn’t actually
intentional by the carrier, but merely a necessary step to ensure
their own services work properly. The blocking affects very few
customers in practice, and it seems that there are currently no
plans to expand the blocking of Apple’s service to standard
Impacted customers will receive one of two error messages, shown
below (sourced from the same internal document), stating that
their plan isn’t compatible with the iCloud Private Relay service.
As I wrote in an update to my earlier post, if it’s true, this is fair and makes sense. If you actually want your carrier to filter your network traffic, you have to let them see your network traffic. There seem to be a lot of reports from T-Mobile customers running into this problem who claim not to be using T-Mobile’s content filtering features, though.
Another thing to check: in Settings → Cellular → Cellular Data Options, make sure “Limit IP Address Tracking” is turned on.
Update 14 January: iOS 15.3, now in beta, updates the description for iCloud Private Relay in Settings to make things more clear.
★ Monday, 10 January 2022