When you attempt to send an MMS message, if the recipient is on a carrier that does not support MMS, Twilio will send the message as an SMS with a shortened URL linking to your media. This includes both international carriers as well as unsupported carriers in the US and Canada. These messages will only be charged for the cost of the SMS message.
You may choose to disable this feature. Check out this FAQ to learn how. When disabled, Twilio will no longer convert your MMS messages into SMS text messages. Instead, your application will receive a HTTP 400 response when sending MMS messages to these carriers.