Press the key. 

The number of waiting external calls appears on the screen. 

If you are already engaged in a call and the number of waiting calls exceeds a certain threshold value (overload; contact Customer Support), the LED on the key indicates overload. 

LED off: No callers are waiting.

LED is flashing slowly: The number of calls waiting has reached the pre-defined threshold value.

LED is flashing rapidly: The threshold value has been exceeded (overload).