This feature is only available for paid e-bulksms accounts sending messages to supported countries. Some supported countries have additional requirements like going through ap re-registration process (see the list of countries which require pre-registration) or only allowing messages that are transactional in nature (one-time passwords, account notifications) to be sent using an Alphanumeric Sender ID.
Alphanumeric Sender ID messages sent to an unsupported re-registration required country will fail sms delivery. In this case, we recommend that you contact us by email or chat with our support team to get help.
Additionally, recipients cannot reply directly to messages sent out using an Alphanumeric Sender ID.