Hello, all SMS messages are sent through AWS SNS. If you are sure that the number format is correct then it might be temporary issue with the local carrier. Sometimes it may be throttling issue and the SMSes will come late, sometimes they get stuck and they are not delivered at all. There is not much we can do about it as the problem is between the AWS and local carrier.
As a workaround we recommend to send the message to phone and email at once. Client can then use the link from message that he receives first.