We have a project to improve SMS API error handling so we can return more accurate error messages.
This is a longer term project which can be broken down to smaller parts, the first of which has just been released.
This first part is a bug fix where if we can’t deduct credit or charge you for the messages then we provided an “insufficient credit” error.
With this release you will now receive “internal error”.
If you do receive “internal error” then we recommend you retry after one minute.
You can read more detail about error handling on this page of the SMS API Documentation.