What is the firmware version on the phone?
Please take a look at this PSN (Product Support Notice). It was published a while ago though so you might have a newer version of firmware on the phone
PSN002963: 9600 IP Telephone will be stuck on data VLAN when upgrading a phone from the factory having 3.125 firmware on it to another 3.1-based firmware release
http://support.avaya.com/css/P8/documents/100092537
Arbi