NEC ETJ 16DC-1 Telephone User Manual free download

In the link

https://www.google.com/search?q=nec+dtu+16d-2+turn+on+voicemail&

Manual

Dtu-16D-2 Tel - NEC Electra Elite IPK II System ... - ManualsLib

https://www.manualslib.com > manual > Nec-Electra-Eli...
NEC Electra Elite IPK II Manual Online: Dtu-16D-2 Tel. This digital Multiline Terminal has 16 programmable line keys (each with a 2-color LED), ...

• 
  

  NEC 16D and 8D Quick Start User Manual - ManualsLib

  https://www.manualslib.com > ... > Telephone
  View and Download NEC 16D and 8D quick start user manual online. Multiline Telephone. 16D and 8D telephone pdf manual download. Also for: Dth-32d, Dth-16d, ...
  
  

..

http://ipora.info/R/O/NEC~ETW~16DC~2~MANUAL.PDF You can download the manual here.

you probably have an ext unit please try the following steps
To set the time, Press FNC then 9 then # then set the hr and mins using number keys Use RECALL button to set AM & PM Press FNC again to set.
Good luck and best regards
John

ringer don't ring incoming calls

just found a site a downloaded manual
but thanks anyway for the follow up

hit feature key, then dial 9 # from the operators phone

hit feature key ,dial 1

The telephone instrument you have can be used on the NEC Electra Professional Level I, and the Level II systems (and under some conditions the NEAX 2000 series). Codes for features are somewhat different based on the system. It is possible that your phone is in "Do-Not-Disturb" if other phones on your system actually ring when they are called via the intercom (on the Level I and Level II systems, initial intercom ring is a system wide setting). The cancel DND code is: Level I system: with telephone idle, press <FNC>, dial 60, press <FNC>. Level II system: with telephone idle, press <FNC>, dial 69, press <FNC>.
It is also possible that your ring volume is very low. To adjust, press <SPKR>, dial 601, then adjust volume using the up/down keys at the bottom of the phone (press <SPKR> when finished. If your system is programmed to access an outside line when you press <SPKR>, you will need to press the <INTERCOM> button first.)
If your system is set for voice intercom, people calling your phone can make it ring by dialing a "1" after your intercom number.
If you do all this and your phone still won't ring on incoming calls, let me know and we'll have to go into system programming to correct it.

I found a good site to download a user manual for any NEC phone system. Here is the link:

http://wedophones.com

Not sure if I understand your question correctly, but I think I just learned how to do that on an electra professional. Memory block 4-1-1 lets you go through each of the internal extensions, and you can set each extension to ring when a call comes in from any/some/all of the outside lines.
to reach memory block 4-1-1, I used a multi line terminal connected to ports 1. Press fnc => hold => # => 0 => * => line key 4. Use the fcn(?) key to cycle through the internal ports, the line key buttons to map each of the 4 lines (presumably line keys 1 through 4 on your system).
As to where to get a manual, I am not sure, I have 3 thick binders... good luck