We currently have an application running in production where we send out 150-200 SMS messages once every week to provide our contract workers a link to a form where they submit their time availability.

We have run into an issue where some of our contractors are not receiving the messages that were sent out. Checking our logs, it shows they were sent a message, it was delivered, and read. They were able to receive a message when I made a single API call to the SMS endpoint, but do not receive them when the messages are being sent out from our application.

We have seemed to uncover that the majority of people not receiving the messages are T-Mobile/MetroPCS users. We took the time to consider carriers filtering out our messages, which is why we built in a 6 second delay between sending each message to throttle the application some. If our messages are still being eventually dropped by the carrier, specifically one, is there anything more we can do? Also, if they are being dropped at the carrier level, but showing sent, delivered, and read from RingCentral, how is it going to be possible for us to troubleshoot who is and isn't getting our messages?

We can't be the first who has experienced this type of issue, so I am hoping someone has advice on what we could possibly do/change.