I installed the wireless adapter on a SPA504G phone. When I connect to the UC320 via wireless, the voicemail button does not function and I get feedback in the speaker. If I make a hard connection to the UC320 the voicemail button works fine.
Make sure that you have configured the wireless adapter to associate with the VOICE SSID configured on the UC320W and not the data SSID. You will need to do this manually. The WPS of the UC320W is associated with the data SSID in case you used WPS to configure the wireless adapter.
Do you have the same problem if you call the voicemail pilot number (default is ext 395)?