Red light flashing on what exactly? If you mean the AKD-5103 single line phone, then it has message waiting turned on either from the voicemail or another set... Try accessing the voicemail first, and if it doesn't have voicemail, look in the FAC's for the Cancel Message Waiting code.
Doing a `LOCATE FEATURE EXTENSION xxxx` may hhelp as well, it will give you more information on active features.