If your account balance reaches zero, you won't be able to use Plivo to make calls or send messages. Your phone numbers, however, won't be removed from your account right away.
You can recharge your Plivo account from the BIlling page of the console. First, make sure that the credit card information in the Payment Details section is complete and correct. You can set up automatic recharge in the Auto Recharge section of the page. Select both a minimum amount at which your account should recharge, and a recharge amount. When your account reaches the minimum amount specified, Plivo will automatically recharge the selected amount.
Alternatively, you can manually recharge your account by using the Recharge section of the Billing page and entering a recharge amount of at least $25.