There is an interesting bug in a Mitel's servers for Voice over IP that allows to discover the numbers called and the numbers calling trought this dhcp server. This server is configurable via http interface and via telnet; in this case, if there is a call at moment of login/pass request, I've noted this:
Trying 192.168.1.2...
Connected to 192.168.1.2.
Escape character is '^]'.
Username: mitel-cs018
Password:
ERROR: Invalid Username/Password pair
Username:
Password:
Username: ^X^W^E^Q^W
Password:
ERROR: Invalid Username/Password pair
Username: Password:
ERROR: Invalid Username/Password pair
# in this moment a foreign call arrive from outside