"flash busy" message when trying to retrieve voicemail messages

Before you can do anything you need a M7310. Programming a Nortel voice mail requires M7310. It uses the 3 soft keys under the display to program it. You can get one cheap on EBAY. Next you need to find out what type of Voice Mail System in connected to your Nortel phone system. Below is a link to a Older StarTalk manual. You may have a StarTalk, StarTalk Flash, or a Call Pilot.
http://www.myuserguides.com/Nortel_Norstar_Startalk.pdf#page=15&zoom=auto,-69,407

Below is a link that tells how to reset the System Manager Password.
Reset Admin Password Norstar Meridian and Nortel BCM Help Guides

The default code and password for System Admin is F983 and 1020000 or 1021111. The photo below shows a system admin programming tree. Hope this helps
Charlie

"Q: My Norstar Startalk, Flashtalk, or voice mail says my mailbox is not accepting any more messages. I deleted all my messages, but callers still cannot leave a message. Why?
A: Check with your voice mail administrator. Chances are another Mailbox on the system used up all the message storage space. Usually it's the general delivery mailbox that hasn't been checked for messages on a regular basis."
Frequently Asked Questions

The following discussion suggests that voicemail can be inactivated, and cab be re-activated by logging in and pressing '4'
Startalk Flash Won answer Nortel Phone System Tech Support

This says that the mailbox must be initialized.
http://www.uniquecomm.com/docs/Startalk_Flash_Quick_Reference_Guide.pdf

Not to be too obvious, but did you mean Feature*98?
Even then, it should be F*981, or another in that group:

Voicemail Feature Codes Norstar Meridian and Nortel BCM Help Guides

press feature #65

The M7310 will display "Message for you" on the LCD screen when you have a message, provided that the Voice Mail system is configured to provide indication to the phone.

The exact steps to do this will depend on what kind of Voice Mail system you are using on your Nortel PBX.

If none of your voicemail feature codes(f980-f989) are active, then that means that the system doesn't recognize any nortel voicemail on the system.Which means that a) your system has a non-nortel voicemail system attached to it, b) theres no voicemail attached to the system at all or c) the voicemail is no longer functioning.

Could be two things, there could be a general delivery mailbox attached to the system which is mail box 100, or the new service provider has a off premisis voicemail attached to the lines. if there is VM on the lines you should get a stuttered dial tone on the line indicating that you have a voice mail.

There is no History button that I know of ~ on the system...

But - the message waiting indicator is not working?
Ok, try this to fix the message waiting... leave a voicemail for yourself or get someone to leave a message. FEATURE 1 and your extension...

-> if the light is Not flashing try FEATURE #806 this shuts off static time and date, for some reason this can shut off the message waiting indicator.

Is your voicemail systems built into the Norstar or is it provided by your phone carrier?

The mailboxes don't need to have saved messages, to be full. More then likely what is happening is that when people call in, they are getting a generic/Auto Attendant vm box, and that one is full. Try checking vm box #100, or #10. That is the Nortel standard general delivery vm box. Please let me know is this helps, or if you more help.