Point a browser at http://10.1.10.1
But you will need to know your uc500 user id and password to access this CUE GUI. The Reseller who installed your system knows this.
Find your voicemail box and set the credentials as you like. Save it and try again ( you shouldn't have to unplug and reconnect your phone but try that if it doesn't work immediately)
You may also ask your Cisco reseller about the Office Manager GUI that is intended as a end user admin tool, which can also do this modification.
As long as the uc500 system was configured with the CCA Partner tool, then OM will work well.