Not able to send messages through channelType - SMS in Pinpoint

I am using the Pinpoint service in Nodejs to send notifications through Email, SMS, and Push. I am currently using the service in the sandbox environment - with some pre-verified numbers as destinations. Although the campaign is executed successfully, I get all the emails correctly but not the SMSs (working for a number and not working for some or working once for a number, etc). The addresses, channels, and everything are set perfectly. Kindly let me know where to look for the issue and what can be done to fix this.