Don't know what type of control unit you're using, but would guess it's an KX-TD816 or 1232 if you're using 74xx phones. Those systems can't pass CID to SLT devices (like a CID box) without a rare and expensive add-on module. And that's assuming you also have a CID card installed inside the cabinet.

Without that module (it's so rare I can't even remember the model number), the only way to make that CID box work would be wiring it ahead of the system. Assuming you have a CID card installed in the control unit (can you see CID on other display phones?) I'd recommend just replacing the 7425 phone with a display model. Would be a lot less work and expense.