The Message SID is the unique ID for any message successfully created by Twilio’s API. It is a 34 character string that starts with “SM…” for text messages and “MM…” for media messages.
To find the SID for a specific message in your project’s messaging logs, log into your Twilio project, select Monitor > Logs > Messaging. Click on the message of interest to bring up the details page for that message.
The message SID is the first item displayed:
To find the SID for a specific message via the API, make a GET request to the Messages List Resource. This will return a list of messages associated with your project along with details about each message. Each message resource will have a “Sid” property containing its unique Message SID.