If you enable the Intercom feature in Advanced settings of a snom telephone configuration, any SIP caller may evesdrop your room by forcing your telephone off-hook!
This intercom feature is supposed to be secured with a <domain> setting in the SIP Call-Info message, but this <domain> string can be currently anything, it seems to be ignored by the snom phone. at least if the invite message comes from a registrar that the phone has successfully registered with, for example your asterisk box.
In the snom configuration the intercom feature is set to "off" by default and you will hear a short beep when the phone gets off-hook and the display informs about the incoming connection. Still, it would be wise to double-check your VoIP-connection to the outside world to make sure this auto-answer feature cannot be invoked by anyone from the internet!
This has been observed with SNOM 190 phones (Firmware 3.60x) and SNOM 360 phones (Firmware v5.3).