when i see that i have a voicemail, i dial 1, log in and listen to my message. I then hit 7 to delete the message and hang up,. my phone still says i have a voicemail. I log in again, listen the voice telling me i have no messages, and when i hang up, the indicator is off. this happens every time.
My advice is to contact tech support to make sure you have the correct voicemail for the type of device you are using. They may also have to reinitialize your voicemail. Make sure to save any voicemails as you may lose them when voicemail gets reinitialized.
Hope this helps