You are probably seeing an old pre-voicemail feature called "Message Reminder."

There will be a code somewhere called "message reminder code" and you can dial this code from the phone (possibly 633) to delete the reminder. Once you have done that, delete the message reminder code from the system, to stop people accidentally setting message reminder.

You say that you've already checked for voicemail messages. I assume from that that you've already left a message and then deleted that message. I assume that you've also already checked the voicemail itself for a locked-up notification port.

If anything I've said did not make sense to you, or if you don't know what how to do it, I recommend that you call out a service tech, whether on a service contract or not. The one hour of service, plus showing you the codes to clear lamps, will be well worth the money.