If a messaging channel receives a delivery error during submission, or if the delivery is cancelled because a successful delivery event did not occur within a specified amount of time, you may or may not be charged for the undelivered message. This is because some channels charge for message submission, while others charge for successful message delivery.
Because the Conversation API is a multi-channel solution, you can configure a failover strategy for your app by prioritizing your Conversation API app's channels or by defining a failover channel in a submitted message. With a failover strategy in place, unsuccessful message delivery on the primary channel triggers the delivery of the same message on a fallback channel. If the message is delivered successfully on the fallback channel, you are charged the normal rate associated with that channel.
Note: Charges incurred depend on a variety of factors which will be covered within your channel contract and associated rate card.