Number blacklists are lists of numbers that are specifically noted in the TextPower system as having all messages to them blocked. The lists are created by each carrier individually and sent to SMS gateway operators such as TextPower who are supposed to block them. Carriers block numbers for various reasons and TextPower can also intentionally block numbers.
The most common reason for a number to end up on a blacklist is because the number has been reassigned to another carrier, which occurs, for example, when a subscriber changes to another carrier's service. Often when a carrier sees a service disconnect on a number, it will blacklist the number. Some carriers add brand new numbers to blacklists to be sure that they do not get spammed.
Reasons are not clearly stated by carriers but the assumption is that they want to avoid trouble reports from numbers that they no longer handle. It is also a way to prevent SMS text spam to new numbers.
FIX: As soon as the subscriber sends an MO message to a short code from their mobile device they will most likely be removed from any blacklists. If they have been assigned a "short code block" (most frequently by Verizon Wireless) please contact us to have it removed.
This indicates that the number now (or again) wants to send and receive text messages. Since all short codes have the keyword HELP defined on them, just have them send HELP to your short code (or to TextPower's shared short code if you are using one).
SMS Gateways are now required to support blacklists. However, since the requirement is recent, implementation of carrier blacklist enforcement is still spotty. Some gateways enforce it, some do not and some enforce it for some carriers but not for others. This is temporary. Eventually enforcement will be uniform. TextPower is fully compliant with all carrier requirements and thus will not send messages to blacklisted numbers.