Which Inter-Tel system are you referring to?
On the Axxess? It's done via Call Routing Tables. The system rings into the CRT and the table process the lines in order. If it gets to a number pattern that you entered, it handles the call based on the command. For instance, If someone calls my house, and the caller ID is an 800# or other telemarketer type call, they are disconnected.
If it's an older system like a Premier, you'll need an extra piece of hardware.
