Ghost calls are random direct SIP calls which make your IP phone ring with a strange caller ID, but the calls don't actually show in your dashboard or call history and when you pick them up there's nobody there. You can read more about ghost calling in our FAQ here. 

Most IP phones have a simple setting which you can enable to stop these - and Snom IP phones are no different. You'll need to access the phone via the online GUI and you can do this by finding the IP address of the phone via an IP scanner, such as Advanced IP Scanner, or through the phone's menu by clicking Menu > Maintenance > System info. Once you're greeted with the webpage, enter the default username admin and default password admin. 

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  You’ll need to navigate to Advanced > QoS/Security and enable the option “Filter Packets from Registrar”