From time to time you may very well have some filtering issues by the Carriers that are not reported to us. There is no standard way practice for carrier filtering and depends on each carrier. For some, filtering can range from a simple static list of prohibited terms to advanced machine learning systems that work in real time. Regardless of the system, carriers keep their filtering systems closely guarded secrets. In turn, Betwext cannot say definitively how these systems work or why a particular message was filtered. We also know that Carries often do not favor repeated MMS sending at times when out of service areas or when there is limited storage/data access. This is unique to each cell phone/location and is 100% on the carrier as we've released the messages to send from our system.
One of the biggest red flags for the carriers is NOT adding opt out language like "reply STOP to unsubscribe" on your campaigns. Carrier won't tell us when their robots pick up this content missing from your text but they will randomly filter when/if it is not included.
Please note that adding "STOP to end" on each of your campaigns and in all of your auto-responders is your best way to be sure you're staying in compliance and to avoid filtering.
Here are some articles to help and may explain more.