When you attempt to send an MMS message from a Twilio MMS-enabled number or short code to a recipient on a carrier that does not support MMS, Twilio will convert the message to an SMS with a short URL linking to your media. This includes both international carriers as well as unsupported carriers in the US and Canada. These messages will be charged for the cost of the SMS message instead of an MMS. The message will still have a Media Message SID (starting with "MM") instead of a standard Message SID.
This feature is called MMS Converter, and is enabled by default. You may disable it account-wide in your account's SMS Settings, or enable/disable it on the Configuration page for a specific Messaging Service.
When disabled, Twilio will no longer convert your MMS messages into SMS text messages. Instead, your application will receive a HTTP 400 response when attempting to send MMS messages to unsupported carriers.