Failed status indicates the mobile handset has not received the messages. If you have entered the mobile number in the correct format the most common reason could be one of the following;
- SIM card is not in the phone
- Out of network range for more than 72 hours
- Have not paid phone bill
- Travelling overseas (connected to another network)
- The number is invalid or disconnected. You can safely delete the number from your list.
For more information read article Understanding Delivery Status Of An SMS