hey, @vivekkr0611
A question from my side.
With your 8 pro (android 11?), would you go to settings > wifi and network > sim and network > preferred network type and see if it is on 2G/3G/4G/5G (automatic)?
If it's not too much to ask also go straight to Sim 1 (if that's where your t-mobile sim card is) and see if VoLTE and calling via WiFI are enabled?
(Sometimes you can use the above options to call in places where there is less coverage and / or if there is wifi available)
Of course, have you already rebooted the phone?
You can in extreme cases also reset the setting of the mobile data, calling, wifi and so on, this can be done (in android 11) settings > system > reset options > Wifi, mobile and bluetooth reset.
The above translated with a translator.