There are several reasons you will be debited for your Inbox usage.
These will be listed on the Inbox Billing page, available to the Subscriber or the Delegated Inbox Admin:
1) Channel Enabled - When enabling a channel with an ongoing monthly cost, such as SMS.
2) Channel Renewal - When a channel, such as SMS, with an ongoing monthly cost renews.
3) Outbound Channel Message - When a practice sends a message to a lead.
4) Inbound Channel Message - When a lead sends a message reply to a practice.
5) Unable to display reason, contact DenGro support - If we are unable to determine the debit reason (highly unlikely to appear and used as a fall back only).