In order to use the SMS feature, the phone number must have the sufficient credit balance. To buy the phone credit follow the following steps:
Step 1: Go to Settings.
Step 2: Click on Phone Settings.
Step 3: Click on the Buy Credit Button.
Step 4: Select the phone number you want to add balance to.
Step 5: Choose a credit balance or enter a custom balance.
Step 6: Accept the terms and conditions and click on the Buy Now button.