This could be caused by there being a block on the number you are sending from, please check with the network that there is no block.
Please also try sending an SMS from a different number to see if that is then delivered.
Please provide us with details of the following:
1. The number you are sending the SMS message from
2. The network you use
3. The number you are sending the SMS message to
4. If you can also let us know the network that the person is using whom you want to send the SMS to.
Please send this information to our Support Desk so that our Technical team can investigate this with our providers.