sms – 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?