As per your concern, we would request you to kindly perform the following steps to resolve your issue.
Check Settings: If mobile internet is not working during the call, then check settings 'Use internet during call', Please follow the below steps to enable 'Use internet during call'. Settings> Mobile Networks> Use internet during call> Enable it, if disabled.
Check Data Roaming: Settings> Connections> Mobile networks> Data roaming to enable it, if disabled.
Reset network settings: Settings> General management> Reset> Reset network settings> Reset settings> Unlock your screen> Reset.
Check another network operator SIM: If the issue has not been resolved, then get in touch with the network operator and try to check with another network operator SIM. Note: kindly contact your network service provider once to make sure there is no issue from their end.
Update device software (If available): Settings > Software update > Download and install > Checking for software update > Software update available > Download > Downloading updates > Install now.
In case the issue still occurs kindly visit the service center for a physical inspection of your device. You can locate your nearest service center with this given link: https://www.samsung.com/in/support/service-center/
