SMS Text Messaging: “Delivers as a spam message.”

I have a custom Cognito Lambda function that generates a random code and sends it directly to a phone number.

try {
    if (!event.request.session || !event.request.session.length) {
         secretLoginCode = randomDigits(5).join("");  
      
      if ("phone_number" in event.request.userAttributes) {    
        const input = {
          // PublishInput
          PhoneNumber: event.request.userAttributes.phone_number,
          Message:
            "Ingresa a tu app con el siguiente código: " + secretLoginCode, // required
          Subject: "Tu código",       
          MessageAttributes: {
            // MessageAttributeMap
            "AWS.SNS.SMS.SMSType": {
              DataType: "String",
              StringValue: "Transactional",
            },
          },
  
        };  
        const command = new PublishCommand(input);
        const response = await snsClient.send(command);
      }   
    } else {     
      const previousChallenge = event.request.session.slice(-1)[0];
      secretLoginCode =
        previousChallenge.challengeMetadata.match(/CODE-(\d*)/)[1];
    }
    event.response.publicChallengeParameters = {
      email: event.request.userAttributes.email,
    };
    // Add the secret login code to the private challenge parameters
    // so it can be verified by the "Verify Auth Challenge Response" trigger
    event.response.privateChallengeParameters = { secretLoginCode };
    // Add the secret login code to the session so it is available
    // in a next invocation of the "Create Auth Challenge" trigger
    event.response.challengeMetadata = `CODE-${secretLoginCode}`;
    return event;
  } catch (error) {
    console.log(error);
    if (error instanceof MissingFieldError) {
      return {
        statusCode: 400,
        body: JSON.stringify(error.message),
      };
    }

    if (error instanceof JsonError) {
      return {
        statusCode: 400,
        body: JSON.stringify(error.message),
      };
    }
    return {
      statusCode: 500,
      body: JSON.stringify(error.message),
    };
  }
}

The text message is consistently being flagged as spam. How can I change this?