In order to use all the Plivo services without interruption, you need to upgrade your account to standard. You could do so by doing a minimum recharge of $25 once you have added your card.

You can setup auto-recharge on your account. Refer to this for enabling it on dashboard and this for threshold details.

Please check the below link for the available payment options at Plivo:

In case of further assistance, please contact the support team.