SMS messages been blocked

Hi,

We provide automated notifications (thru SMS) on behalf of our customer. These messages are NOT marketing messages at all. We use Plivo (an SMS service provider) to send our messages.

For some reason since yesterday morning, all Cingular Wireless/AT&T stop receiving our messages. All other providers works fine, but CA/AT&T customers.

Is there a place to register our number so all of our customer can received the SMS messages?

Thanks,

Luis