I have a NEC Dterm80 phone system

Outside callers, only get an operators voicemail, when your phone can't be reached. For whatever reason. No contact to a cell, switched off, airplane mode, you name it.
Then when you have contact to the provider, you should setup your voicemail, the way your provider would like you to do.

Have a coworker call your extension to see if DND is on, they will go straight to voicemail or get a fast busy. Speaker 42 may be the default cancel DND code, depending on your system and whether the codes were changed. Anyway, ringing is set up in system programming. If someone calls your extension it will likely ring, but outside calls have to be assigned to ring in system programming. Usually need a tech to do the programming since it gets complicated very quickly.

You can forward all to another extension or to an outside number. You can also forward busy/no answer to both of those, but then it'll cancel your busy/no answer to voicemail. Speaker 41 plus ext or 9 plus outside number will set CF ALL, speaker 43 will set CFBNA, 42 and 44 to cancel. These are the default codes for NEC pro II, Elite, and IPK. The DTP phone also works on other systems that have different codes.

This is a transaction box (like mailbox but does not take message)...You need the mailbox number & pass word. If you do not have notes on the system configuration you will need to get support from your vndor

I believe your landline carrier just needs to have the capability of call forwarding (ie we utilize Verizon for our business). We also use the call forward feature by: picking up line, (hear dial tone) dial *72 [ then phone number you want calls to forward to, even a cell]. To discontinue: pick up line (hear dial tone) dial *73

To here ring types hit speaker then 711, then 1 for internal rings (select 2 for outside ring selection) then 1-8 the ring type.
To set ring type hit speaker then 720 (follow all the steps above).

The call forwarding was canceled; try this
1. pick up handset or press speaker
2. dial the code for call fwd busy/ no answer (default code is 744)
or press the forward function key
3. dial 1
4. dial the extension (lead number of your voice mail)
5. press speaker or hang up

Do a search for a plantronics headset. All of theirs work well with NEC

try pressing the ext number then 7

I have the 8 button phone and if you hook up your headset the right way you just have to press the line/feature access key all the way to the right instead of picking up the receiver. Keep the headset on all the time and just press that button to answer or hang up the calls. It works great! ( I have the Dterm80 telephone)