Wireless carriers reserve the right to regulate traffic traveling across their networks.
Carrier filtering is the practice where carriers block all or part of a message from being delivered to its intended recipient. Each carrier has its own set of policies related to why they filter a given message, however, these vary greatly are not made public.
When a phone number in our system is carrier filtered more than once in a day, our system will automatically remove it from the pool of outbound numbers being used. The system performs this way to prioritize message deliverability above all else. When text message gets filtered by the carrier, the likelihood that this phone number is filtered again increases.
Carrier-filtered numbers are released programmatically 30 days after they have been swapped out. During that 30-day period, we will still route any inbound messages to their intended destination.
We recognize that messages coming from multiple numbers can be a little confusing, but with carrier filtering, it's possible that messages will not go out at all if we stick to one number.