Plivo is a pay-as-you-go API platform. Plivo relies on its extensive carrier network to deliver messages successfully. In some cases, messages may be sent by Plivo that are not received by the end-user. Plivo charges for messages that are successfully 'sent' out of the Plivo platform.
Do I get charged for inbound messages when the message_url or message_method is not specified?
Yes. Inbound messages are free for long codes but are chargeable on toll-free and shortcodes, even without the message_url or message_method configured. In this case, the inbound message state will be set as received.