Why is SMS not being delivered to certain carriers in Production only?

In our Sandbox account, we are able to successfully send SMS messages to all carriers we tried (Verizon, AT&T, Cricket, T-Mobile, Sprint, and others) as well as Google Voice. When we switch to using the Production account/API, SMS messages to Google Voice and Verizon continue to work, however, both AT&T/Cricket and T-Mobile do not receive the message. The status code in the API responses for all of these messages is 200 and does not indicate any errors. We've noticed this problem for at least a couple of weeks. Is there a current issue with the SMS Gateway?