You can show the number of external waiting calls on the display by pressing the "View number of calls" key . 

Press the "Waiting calls" key. 

If the number of waiting calls exceeds a preset limit while you are engaged in another call (overload), the LED on the key lights up. Contact System Support to find out the waiting call limit. 

LED off: No callers waiting.

LED flashes slowly: You have reached the programmed threshold.

LED flashes rapidly: You have exceeded the threshold value (overload).