Receiving the validation SMS can take some time depening on the User country and mobile network. In some instances it is not possible to deliver the validation SMS.
The user should wait for a incomming call, if a validation is not completed through the SMS text a automated call will be made delivering the validation code to the user.
If a user does to many unvalidated verification requests a timeout for the specific number will be set. This means a user should wait 10 minutes to try again and wait for the automated incomming call.