Could you tell us your current firmware version? Settings > System > About phone > Build number
Do you mean that the problem appears when calling with the Phone app or other third party apps as well?
When calling, is the volume low when only in handset mode, or do you use earphones, bluetooth, speaker mode?
When the Bluetooth volume is low which app, version are you using?
Could you tell us the brand and model of your Bluetooth device?
Thank you 🙂