Use the AWS.MM.SMS.OriginationNumber and AWS.SNS.SMS.SenderID parameters to choose specific origination identities if a particular Region has multiple origination numbers or sender IDs.
Note: Using the AWS Lambda function to send SMS messages has cost implications, based on the Lambda pricing model.
Default preferences for origination numbers
When the origination number isn't specified, AWS uses the following preferences to send the SMS message:
AWS checks if the account was assigned a short code. If yes, AWS uses the short code to send the message.
When there's no short code, AWS selects the available long code associated with the account and the AWS Region for that particular country. For the US, if there's no short code associated with the account, AWS selects 10DLC as a preference over the toll-free number.
Or, you can make sender ID the default in SNS-SMS text messaging preferences through the SNS text messaging console or the SNS SetSMSAttributesAPI. Amazon Cognito uses the configured default value of the sender ID as origination identity when sending SMS messages.