Call Automatically Disconnects When Calling

I have a **android tablet with android 5.1 lollipop** in which I have an **airtel number**. When I call someone using my number (not WhatsApp just normal call), sometimes it shows "**Dialling**" and then automatically call ends in 1-2 seconds. Sometimes, it directly disconnects in 1-2 seconds. Last thing I remember was that I was playing with some of the dial codes of Android. From then this problem started. Before that, I was able to make calls with the SIM which I have now & also with my old SIM