Flight mode let's calls through

[Satguru]

I have a new S21 and the flight mode does nothing and appears to be a fault with the phone. Can this be fixed or will it need replacing? 

[BandOfBrothers]

That's weird @Satguru 

Turn Flight Mode Off then soft reboot by turning the phone off and on and then turn flight Mode back on.

Put the phone in Safe Mode if the issue continues which disables downloaded apps but leaves the stock apps working and then see what happens.

https://www.samsung.com/uk/support/mobile-devices/how-to-start-my-galaxy-device-in-safe-mode/ 

If this then still continues then a Samsung Experience Store  / Samsung Service Centre can help and or have it swapped out by the source that supplied the phone.