This is most likely caused by your voicemail box being full.
You will need to delete the messages off your phone. If you wish us to do this for you please contact our Support Team for assistance.
If you are using your phone to play back your voicemails you will need to do the following to delete the voicemail messages:
Dial *98 from your phone's handset.
Enter your mailbox number (this is your extension line)
Enter your password - this can be found inside the CRM Phone - Line Configuration settings
Follow the voice prompts to listen to your voicemail messages.
After each message press '7' to delete the message.