Message waiting indicator light will not turn off keeps flashing

https://www3.bellmts.ca/mts/support/phone+services/troubleshooting/you+have+no+new+messages+waiting+but+your+phone+says+you+do

Turn off Message Waiting Notification temporarily

1. If you know you have a message waiting, but do not wish to retrieve it immediately, you can turn off Message Waiting Indicators.
2. Press *93 to turn off the flashing light and stuttered dial tone until another message is received in your mailbox.

From att.com:

Q: What should I do if I my Message Waiting Indicator, stutter dial tone or light on my phone, is on but I have no new voice messages?
A: Make sure to check all message types when reviewing your messages (i.e. voice, email and fax).
Q: What should I do if my Message Waiting Indicator (MWI), stutter dial tone or light, is stuck on?
A: Power cycle (turn off/on) the Phone or Customer Premise Equipment (CPE).

That is the message waiting indicator, it is telling you that your Call Notes (Or whatever your service provider calls it's voicemail) has a message waiting. If you do not have a message waiting, you can turn the light of by following these steps:

Resetting the voice message waiting indicator
When you receive a new voice mail message, the new message LED on the top of the handset flashes and
appears in the handset display. In the event your message waiting light gets out of sync with your phone company’s
voice messaging system, you can manually reset it back to the “No messages waiting” state.
Press and hold FIND on the base for five seconds.
Note: The handsets connected to the base will start to beep, however, keep pressing for five seconds.
A beep sounds and the message alert tone is reset.

The term we used for this back when I worked for the phone company was a "stuck message waiting indicator" and the way to fix it was:

Call yourself and leave a message.
Hang up and wait 2 minutes (we used to say 5 minutes, but 2 is plenty)
Retreive/delete your voice mail message
Hang up and wait 2 minutes


That should clear the blinking light.

This feature is country dependent and may not
appear in some countries.
If you have subscribed to the voicemail service
from your local service provider, icon turns
on when you have a new message in your
mailbox. After you have accessed to all the new
messages, icon will turn off automatically. If
you find that the message waiting indication
cannot be turned off by the network, you can
still turn off the message waiting indication with
your phone by the following step :

In idle mode, if voice mail waiting icon is on,
long press the redial key to turn off the voice mail waiting
icon.
Once you choose to delete the message waiting
indication, all the other handsets registered to
the same base station will turn off the message
waiting indication accordingly.

Press down the "find handset" button until the paging beep stops (about 5 seconds) and this will reset the indicator lights on the handsets.

go to the phone where the light is flashing press intercom #409. you may have to do that more than once. Keep doing it until the light goes out.
If the phone system is a DK 8 orDk16 press intercom 409 this will cancel the message waiting light that could have been cativated by a different extension.

The term we used for this back when I worked for the phone company was a "stuck message waiting indicator" and the way to fix it was:

Call yourself and leave a message.
Hang up and wait 2 minutes (we used to say 5 minutes, but 2 is plenty)
Retreive/delete your voice mail message
Hang up and wait 2 minutes


That should clear the blinking light.

First of all here is a link to the manual for you ...enjoy !
http://www.sellcom.com/ET4000%20users%20guide%20%20v7%20090803.pdf

On page 72 there are instructions for re-setting all programmed features to default, then you can start afresh...good luck

Try holding the mute/find handset button on the base unit for about 5 seconds and the light should stop flashing.