SMS.to partners with different carriers and network providers. These providers charge per submitted SMS message. All submitted messages from SMS.to platform to the local network providers were charged regardless of the status. To learn more about the different types of status, please refer to How to Check SMS Logs & Status.
We have no interest or advantage if you get any FAILED messages, your loss is our loss and we ourselves get charged the same amount for FAILED and DELIVERED. If a phone number is recognized as invalid, we do not even submit it for delivery and the message will remain in status REJECTED without any charge for you.
Anyhow, the possibility remains that a phone number has a valid format but is still inactive, without reception, etc.
We recommend our users to utilize SMS.to's HLR list clean-up feature to prevent FAILED messages and keeping the list of numbers clean by removing inactive numbers. To learn more about the HLR list feature, please refer to How to Use HLR or contact us to learn more about how to improve delivery for your specific use case.