Following are the different scenario when the delivery of your messages would fail:

• Number is Invalid and Not Specified in Correct Format – To avoid delivery failures we recommend keeping mobile numbers in the international format. For instance, within the UK the numbers you start with a 0 when calling or SMS (i.e. 01509 813888). However, for an international call or SMS to the UK you need to add the prefix (44) and remove the leading 0 (i.e. +44 1509 813888).
• Carrier Spam Filters – Most carriers prefer to use regulations to control the flow of spam. In developing markets, however, technology is used to filter out unwanted texts and velocity. These filters, however, are not perfect and can lead to filtering out legitimate SMS traffic, as well. Some filters even send back a fake handset delivery confirmation. You think your message is delivered, but it is not. Furthermore, filter technology is in constant evolution, and associated algorithms change constantly. Keeping a track of which carriers filter and how, is a complex task.
• Invalid Sender-ID – Frequently seen when trying to use non-authorized Sender ID in North America, where a long virtual number or short code is required.