If you ever hear the Mailbox start ringing as you walk by it (most likely a single-chime Trimline style ringer) that's a good clue. And the "T-handle" latch added to the door is another clue, that style mailbox usually comes with a flush cylinder lock. This one can be hand opened and latched many times during the day.
The hard part will be getting the "panel phone" with the retracting handset cord for it. (If you can find one, a Western Electric 750B would be perfect - 2750B would be the Touch-Tone model.) A regular curly handset cord will get beat to heck every time someone forgets to tuck it inside before latching the door, and they always do.
--<< Bruce >>--