Thanks. I just called them. They said that VoLTE was not activated on my number and that I'd need to wait for 3 days for the same.I'm also on Stock ROM. IMS Status showed registered for me.
Have you tried Airtel VoLTE on any other compatible device(from their website) with your SIM?
Airtel has a very bad system of activating VoLTE on a per number basis according to your device. It's not the same as Jio where VoLTE just automatically gets activated on a compatible device.
I had to put my SIM in an iPhone and then call customer care thrice to get VoLTE activated on my number. VoLTE did not get activated automatically on my number just by putting my SIM in the iPhone. They did a backend network reset; see if that helps you.
Also, I tried my friend's SIM who uses Airtel on Oneplus which already has volte activated with my pixel but that also didn't work. I will wait for few days and check again.
Edit : VoLTE is working now. Thanks a ton guys. Thanks OP.