When a message is sent, the carriers will give us a response code to indicate whether it was successfully delivered to the device or failed. A failure response may include information about the reason; perhaps the device does not accept SMS/MMS from our system due to limitations from their carrier plan, the message was sent outside TCPA time restrictions, or the phone was out of service or out of area.
iZigg recommends periodically scrubbing your contact list by removing contacts that have failed three or more times in a row. This will reduce the number of credits you utilize for each sent message improving ROI and saving you money. It also provides more accurate reporting and visibility, allowing you to more easily see trends in message delivery and response.