[Cultural Jihad]

I haven't been able to find another caller ID box with the "extension in use" option.

Radio Shack has many such products.

[Drammach]

I keep a large brass Police Whistle by the phone.
When I recieve prank calls, heavy breathing, etc., I simply pick up the whistle and blow REAL LOUD, into the phone.
The prank calls usually end immeadiately.