Sending SMS is a pay per use feature, your plan may come with some credits included but you will need to have SMS credits in your balance to be able to send SMS.
What are SMS Credits
A standard SMS is 160 characters in length and is equal to 1 SMS credit. If your message is longer than 160 characters than the cost increases by 1 credit for ever additional 160 characters you write.
View SMS Credit Balance
To view your SMS credit balance:
Head to Settings
Click on the SMS tab
See your SMS credit balance in the top left of the page.
View SMS Credit Usage
To view your SMS credit usage:
Head to Settings
Click on the SMS tab
See your SMS credit usage from the SMS usage table
