Error 21408 occurs in in twilio SMS API when we have not enabled the country in twilio console to which we are sending SMS.
To enable the country, simply follow the steps given below.
- Login to your Twilio Console.
- Goto the following link: https://www.twilio.com/console/sms/settings/geo-permissions
- Simply enable the countries to which you want to send SMS to.
You can check the APIs to send SMS here.