What is Error 38 and how do you resolve it? My phone is working normally and network connection is fine. While trying to send an SMS, I get a notification for 'Error 38: Unable to send sms'. The message remains unsent after multiple attempts.

(1) There is no detectable interruption in network; I can use internet services, make calls and even receive SMS.

(2) This is not an account balance issue.

(3) Toggling between networks (2G/3G) does not help. Sometimes the issue resolves after a while.

(4) My network is Airtel (India), handset is Xiaomi A1.

Could someone help me understand what could be happening and how I can avoid/resolve this issue?
This is because your outgoing service is barred by your service provider. If you are from India then most of the service providers here stop the services if recharges are not done in proper interval as per company's policy.