I have a red flashing light on a NEC DT400, it is the reception ext key. How do I cancel it out?

Try 751 as the code. You can also try unplugging the main control box at the end of the workday for 10 minutes and plug it back in. This might remove temporary programming.

NEC DT400 Phone Manual

Rhode Island Telephone https://www.ritelephone.com > uploads > 2019/07
To Program a Line key as a Programmable Function. Key: 1. Press the Speaker key. 2. Dial Service Code 751. 3. Press the Line Key to be programmed.
72 pages

I have never heard of this 'hunt group' feature before.

https://www.google.com/search?q=nec+dt400+programming+guide

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Ideas and programming guide in the link

https://www.google.com/search?q=NEC+DT400+voice+prompts

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These instruction are for a Univerge-SV8100
To Delete a Security code, the greeting, messages or recorded name in a personal voicemail box:
1. From any phone with Admin rights (Usually ext 101)
2. Press the message button
3. Enter security code if there is one
4. Dial 72 to access System Admin
5. Press SUBS soft-key (in display)
6. Dial the extension number you want to change
7. Press the available softkey in the display or 7 to delete the security code.
Be careful when you are in the admin part of the programming because you can really mess up the voice mail if you do something wrong. To check what type of phone system you have look for a NEC box in your phone room or server room. The complete PDF is here
http://www.integratelecom.com/resources/Assets/sv8100-admin-inmail-master-11-5-07.pdf

Blinking Light
Two reasons the light on top of your screen might be blinking or on steady.

• You do not have a voicemail key programmed and the voicemail is alerting you of a new message. Simply dial *8 and check your mailbox.

• Message waiting has been activated. To cancel:
  • leave handset hung up
  • press call1 key
  • dial 873
  • press speaker key

Blinking light on top of Display NEC Aspire

.. I don't have a solution, I need a solution.
NEC Dterm Series I, through internal calls my phone flashes notifying me that I have a call coming through but it doesnt ring. I have tried the volume key ^, that didn't work. Can anyone help me?

we are using nec ip2ap-12txd tel2(wh), incoming voice is very low can u suggest how to increase the volume

Another extension may have the line off hook.

from operators dss hit mw key then the ext you are trying to clear

There is no such thing as a "Call In Progress" light on any NEC phone system. If you are talking about the Message Waiting light you will need to handle the message, or cancel the forwarding or do Feature 99 Feature.