One of my contacts (and only one) who shares my account has an icon beside her number that looks like a cross between a text-message box icon and a padlock icon. Likewise, my number on her phone has the icon. These icons appeared out of nowhere. Neither of us did anything to the contact or changed any settings on the phone, and everything was fine before these icons showed up. The result is that neither of us receive the others' text messages.
There is no problem with messaging between the other phones on our account. That we're reciprocally blocked indicates to me that this is a T-Mobile issue and not a phone issue.
Does anyone have any idea what's going on here and how to fix it?