Cell phone users change carriers because they either want to save money or are not satisfied with their current service.
When a phone number is assigned to a new carrier, it is also assigned a new local routing number to identify the new carrier. This process of transferring an existing number to another carrier is called number porting.
If you are sending messages to a specific carrier and if a number is ported, the message will be sent to a wrong carrier. This is why it is important to verify a number’s carrier designations prior to sending text messages. Here is an inexpensive and highly accurate tool that can save a ton of money and headaches: www.SubscriberVerify.com