How do SMS charges work?

This article details exactly how the SMS integration charges for SMS.

The SMS integration will initiate a charge for every SMS that passes through Vonage. This means that even automated messages sent by queues will issue a charge. 

Failed SMS will also initiate a charge. This includes attempting to send MMS messages (which isn't supported).

What about character limits?

For any SMS over 160 characters, Vonage will split into separate messages. For example, if you send an SMS that is 400 characters long, it will be split into 3 separate messages.

You are charged for every SMS (so in the above example, there would be 3 charges).