Twilio's alphanumeric sender ID feature allows sending branded SMS messages using a business name as the sender ID in 145 countries around the world. This feature is suitable for alert or broadcast type messages but not for interactive applications where users cannot respond to the message. When sending an SMS message from an alphanumeric sender ID, if the recipient's country does not support it, Twilio will return an HTTP 400 response with a specific error code (21612), indicating that the sender ID is not supported in that country. To handle this scenario, developers can rescue the exception and retry sending the message using a Twilio phone number as the sender. Using alphanumeric sender IDs enhances the user experience for SMS alerts and broadcasts by branding the sender's identity and allows fallbacks in cases where the receiver is in a country that won't accept an alphanumeric sender ID.