Which Operator You're Using ??
Did You Tried Removing The Sim Card & Re-Connecting ??
Did You Have Mark The Option "Enhanced 4G LTE Mode" ??
I Hope This Will Help
Thanks for the reply.
I am using Vodafone and Airtel in Bangalore.
I tried removing and reconnecting. I tried by swapping sim cards, but nothing worked.
Yes I have enabled "Enhanced 4G LTE Mode" in network settings for both the sims.
First Of All , Vodafone Didn't Rolled Out VoLTE Support For Every Single Nokia 7 Plus Yet . (Android 8.1 V2.22 Can Be Work Out Luckily For Some)
In Case Of Airtel , Did Your Area Mentioned As VoLTE Service Enabled ?? (Not 4G Only & Not Just Whole Bangalore , It Has Limitation For Now)
If Your Answer Is Yes Then Did You Activated Airtel VoLTE Earlier ?? Try To Talk With Customer Service Person Or Visiting A Nearest Airtel Relationship Center Will Be Best, Cause They Can Really Help You About VoLTE Availability .
Let Me Know , If Your Problem Solved .
I have the same problem in Croatia (Croatian Telecom). VoLTE is activated on my SIM card but Enhanced 4G/LTE mode is note visible in Network settings for turning on/off. When I tried with code *#*#4636#*#* in Phone information "VoLTE provisioned" and "WiFi Calling" are gray and cannot be enabled. Before Nokia 7 plus i used Xiaomi Mi A1 and VoLTE worked without problem even Xiaomi devices are not certified in Croatian Telecom. But in settings (od via code *#*#4636#*#*) I was able to turn on/off VoLTE/WiFi.
Vodafone India is having issue on activating VoLTE on Nokia 7 plus. They are working on it.