Where is my "BUSY"?

Hi Guys!!!
Can anybody help me with one trouble?
I have *1.8 + AstriskGUI.
If I dial to busy extension I have a normal dial-tone and called extension have a notice three-party call
I think this is standard feature, but I cannot turn off it.

Hi ALL !!!
The trouble is closed !!!
The problem was in IP-phone Cisco 300.
It’s has got “Call waiting” feature by default ''on".