Ok.. I have found the way to get the thing working. The NEC Electra Professional uses two Ring Assignment tables, one for day shift and another for night shift. In our system also exists a button in the console assigned to change between day/night attendant configuration mode. To acces the configuration for day shift using a ETW16DC1 phone you must enter:

[fnc] [hold] [#] [0] [*] to get the system in programming mode.

once there you press [LK4] (line key 4) and digit 01 in the numeric pad. this brings you to the Ring Assignment configuration. Here you see a number in the top left LCD display starting at 01. this is the port number assigned to the fisrt extension. In our system, port 01 is assigned to x100, port 02 to x101 .. up to 32 ports. you can cycle between ports using the [Cnf] key. the leds in the LK buttons shows you the ring configuration for that port. Off led indicates that line is not assigned, green led indicates taht CO line will ring the phone on that port, and red led indicates same but with a delayed ring. You can change the status of the leds pressing the corresponding LK button to cycle between off-green-red. this way you map the ring form outcoming calls to the ports you want. When you are ready, press the [Trf] button to write the changes and press [Spkr] to exit programming mode. In our system we assigned all CO lines ring on x100(receptionist), x102(human resources), x107(Gatekeeper) and x124(Security) that are the attendants in charge.

In turn you can map the ring for the night shift same way but write 02 after pressing [LK4] key.

Well.. after 1 complete day of reading the NEC manual I think this can be usefull to someone. BTW the setup for this featurfe is located in page 2-233 of the Electra Professional Level II and Level II Advanced Installation Service Manual.

Cya.  =)