Yolla credit may become inactive after 365 days (one year) if you do not use it for a period of 365 days (including the credit that has been allocated to you for free when downloading the app or by a Yolla support specialist).
You can reactivate your Yolla credit by logging into your Yolla account at https://yollacalls.com/en/account/. Reactivated Yolla credit is not refundable.