Your SMS text message may include a URL http://m.twil.io followed by 7 unique characters if you are sending MMS messages to a carrier Twilio does not support for picture messaging.
When you attempt to send an MMS message, Twilio would check if the recipient number is with one of the carriers we support. If not, Twilio will attempt an SMS instead and include a shortened URL in the body as a link to your media. The URL remains active for 7 days.
If you're using Messaging Services, you can disable the "MMS Converter" feature under "Copilot Features" section. To disable this feature on the account level, please write Twilio support.