Have you checked if the Enhanced 4G LTE Mode option in the Mobile Network option in the Settings is enabled? If it is not enabled, then you need to check this option, then it will work fine.
If you do not find an option such as Enhanced VoLTE, then try to change the SIM card to a different slot (it will be better if Jio SIM is in the first slot) and restart the phone and check again if such an option appears. If you do not find the option again, then try to clear the cache partition the method is seen here in the link here.
If you still do not find any solution, then you can finally try the solution below. First, you need to have ADB drivers installed on your PC and you need to enable developer options and debug mode on the phone. Then try using the following commands as below in command prompt (cmd if you using Windows system) or Terminal (if you are using Ubuntu)
adb reboot bootloader
(useful to start bootloader of the phone)
fastboot erase modemst1
fastboot erase modemst2
- Press the lock key to start the phone from bootloader mode