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 SMS and “MM…” for MMS.
To find the SID for a specific message in your account’s messaging logs, log into your Twilio account, select the “Messaging” product, and navigate to 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 SMS messages associated with your account along with details about each message. It would be the “Sid” property in a message resource.