The most likely scenario for this is carrier network congestion during the time of request. This could be caused by the operator receiving an influx of SMS requests or the user being in a low reception area. Ask your end user to retry. In addition, make sure that your end user enters an SMS enabled mobile number with an SMS plan.
If you experience many users with some commonalities, please contact TeleSign Support for further assistance.
NOTE: Single number issues are hard to investigate and address since the operators usually do not consider this a potential outage. Operators are more likely to consider it a one-off situation.