Telephony actions like calls and texts have different costs depending on the number being reached - if you are exceeding the threshold set in Settings, you may get an action that won’t complete.
To check, go to Settings in your Admin Panel - Look in the Telephony Credits section to see what your “Max credits per action” is set to.
Check out the Telephony Rate Card, and find out if the rate could be exceeding the threshold set. Compare this against your Telephony Log
Some countries have “Do Not Text” lists which prevent texts from coming through that are sent by services like Duo. If that is the case, your user would need to get their number removed from such a registry. Some countries like India have reliability issues with SMS. It could also be an issue with a telephony provider.