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