In some countries (Luxembourg, Burundi), there is no national dialing code (0). You need to dial to those countries without the 0.

For those countries, or if you need it from some reason, you can do it by navigating to 'Settings: Servers: National Dialing Code' and set it to 'No'.

Please note that this option will be checked first, before 'National dialing code' on Trunks.