Mobile data goes goes away while on a voice call on iphone 12 pro

Did you already try to set it back to factory settings, set it up manually without the backup. Then test it again, if making calls still use mobile data.

Or are you using a messenger app, that can also make phone calls? In that case you might use your mobile data while on a call.

Restore your iPhone, iPad or iPod touch to factory settings

Both mobile data and Voice Calls are carrier Specific features.