Fair enough, if it was traversing public (non-ATT) networks without a tunnel.

It would be reasonable for AT&T equipment to block any private IP traffic outside a customer's private network by default. That's ignoring that AT&T's network is public, and it wouldn't make sense to use a private IP for a service they provide.

Reversing that would likely require AT&T to make a firewall change to literally every piece of equipment they operate, and that's assuming that they don't use the blocks internally. That, and I can guarantee that some customer, somewhere, would be using whatever IP they chose.