bahram,
Well, if you have access to DB Programming then go to System > Phone-Related Information > Flags > Handsfree On/Off
Remove any extensions you find in there.
Next, go to System > Phone-Related Information > Flags > Ring Intercom Always On/Off
You should really make both of these part of your default programming changes for new phones if you don't want this happening in the future.
Handsfree On/Off is what allows a phone to auto answer on IC calls and Ring Intercom Always on an extension ignores the Handsfree feature flag of any phone that is still turned on and rings the phone anyway.
If you don't have access to DB Programming then you just have to go up to the phone and while it is idle, handset in the cradle, dial the feature code.
If it requires the SPCL key then you will need to make sure that you have a Special Key setup on the phone; default is the Cancel key that looks like an X with a circle around it. The steps would be the same except you would press the SPCL key prior to dialing the feature code.
Thanks,
TE