Carrier support for MMS varies depending on whether you are sending from a long code (10-digit) local Twilio number, or a US short code Twilio number.
In addition, an MMS Converter feature also exists to help ensure that outgoing MMS sent to unsupported networks can still be delivered as SMS.
MMS from Long Code (10-digit) numbers
Currently, Twilio supports sending MMS messages from US and Canada local long code numbers to the following mobile carriers in the US and Canada.
Major carriers: AT&T, Verizon, T-Mobile, Sprint, Rogers, Bell, Fido, Telus, Wind Canada
Minor carriers: 365 Wireless, Alaska Communication System (ACS), Advantage Wireless, Alltel Wireless, Bluegrass Cellular, Boost Mobile, Carolina West Wireless, Cellcom, Cellular South, Clear Talk Wireless, Commnet Wireless, Copper Valley Telecom, CTC Telecom, GCI Wireless, Google Voice, Illinois Valley Cellular, Inland Cellular, James Valley Communications, Leaco, MetroPCS, MTA Communications, NewCore Wireless, Nex-Tech Wireless, Panhandle Communications, Peoples Wireless, Pine Cellular, Pinpoint Communications, Southern Linc, SRT Communications, Standing Rock Telecom, Thumb Cellular, Simmetry (TMP Corporation), United Wireless
MMS from US Short Code numbers
Twilio US short code numbers can also be enabled to send MMS messages to major carriers, upon request. For more info, see Sending, receiving MMS messages with short codes
Supported carriers: AT&T, Verizon, T-Mobile, Sprint, US Cellular
At this time, Twilio short code numbers in non-US countries do not support MMS.
What happens when I send an MMS to an unsupported carrier?
Twilio offers a feature called MMS Converter, which can automatically convert your MMS to SMS when sent to unsupported networks.
With MMS Converter enabled: when you send an MMS message to an unsupported carrier, Twilio will send the message as an SMS with a shortened URL in the body, linking to your media. The URL (http://m.twil.io followed by 7 characters) will be appended to the body of your SMS text message.
With MMS Converter disabled: Twilio will return an HTTP 400 response to your API request with error 21612 when you attempt to send MMS messages to unsupported carriers.
Check out this FAQ for more information.