Update, I was able to resolve the issue after setting from_user and from_domain in the pjsip.conf,
from_user=+9180********
from_domain=ims.airtel.in
Found this FreePbx discussion to be really helpful.
From the logs I noticed that REGISTER call was successful but the subsequent calls were not. Then I changed the settings so that the From header would match what was being sent in Register. That worked!