Currently Twilio supports sending MMS messages to the following mobile carriers in the US and Canada:
Major carriers: AT&T, Verizon, T-mobile, Sprint, Rogers, Bell, Fido, Telus and 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.
When you are reaching a carrier not listed above via MMS, and when the "MMS Converter" feature is enabled, Twilio will send the message request as an SMS with a shortened URL 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. When the "MMS Converter" feature is disabled, Twilio would return an HTTP 400 error when sending MMS messages to unsupported carriers. Check out this FAQ to learn more.