Note: If you have signed up for your account after 24 April 2023, please visit the following article: Adding or Recharging Your Phone Credits

If your customers hear a busy tone when they call, your phone credits could be below $1. 

Please add more credits and if the problem persists contact the support team. 

If you are the admin of your account, recharge your phone credits.

To do this, go to Admin Settings > Account Settings > Plans and Billing > Phone Credits > Purchase Credits

Select the amount you want to add click continue. For details, see How to add phone credits?