AT&T Door Phone Controller Installation And Operation Manual page 37

LED
When the AT&T Door Phone Controller is powered on and
functioning normally, the LED (Figure 1-1) should continually
blink 2.5 times a second. If there is a malfunction the LED will
blink faster than 4 times a second (see Troubleshooting
section) or go out completely. If the LED is not lit, check power
to unit first before calling for repair.
Installation Procedures 2-21