There are a few scenarios where text messages will go undelivered. The most common reasons are a) the phone number is actually a landline that does not accept texts, b) the phone number is no longer in service or is disconnected for another reason, c) the mobile carrier blocked the message as SPAM, and d) the number you attempted to text isn't a valid phone number. Since you and only you are responsible for the phone numbers you enter you will be charged for any attempt to send a text message to a phone number. Under some circumstances our 3rd party texting service can realize a phone number is invalid (ex: area code doesn't exist) before an attempt is made, and this is the only scenario where you will not be charged for an attempted text to an invalid phone number. When the delivery status of a text is returned to us we record it in the database. If any phone number receives 3 straight undelivered statuses we will opt out this phone number in your account so that you we do not continue to attempt to text this phone number at your expense.