You need to manually purchase SMS credits unless you have enabled the auto re-charge in your Simple Salon Messaging account.
You can enable the auto recharge by going to Admin > SMS & Email > click Buy Credits, which will take you into your Simple Salon Messaging account.
Click Cancel in the pop-up window to see your account behind it.
In the Billing Settings section, you'll see the Auto Credit Re-charge setting. If you don't want the auto recharge to occur, then choose the Do not auto recharge option from the drop down menu.
If you want to enable the auto recharge, then choose an amount from the drop-down menu, and this will be the amount that the account will auto recharge you when you have a zero balance or when payments required exceed available balance.
You can also choose to have a balance warning sent. Drop down the Send Balance Warning drop down menu and choose when you would like to receive the email notifying you of a low SMS credit balance i.e. when the balance is less than $5.
Once you have chosen your preferences, click Update.