Unfortunately, there is no way to guarantee a message will be received; text messages rely on carriers for delivery.
However, because of the high response rate to text messages, you can infer that the vast majority arrive. You can see success rates by clicking on individual campaigns from the Messaging tab. Common reasons for a success rate being less than 100% are landlines or disconnected numbers.
Can I See Which Numbers Didn't Receive My Text Messages?
If you view the list your campaign was sent to, you'll be able to see the numbers that have been marked as invalid. This means we got a response back from the carrier that we are unable to deliver a text to this number.
As mentioned above, common reasons for this are disconnected numbers or landlines (in the case of imported numbers).