Yes, you can configure an user with extension xxx and voice mail enabled at Users page of Configure Utility. You don't have to assign a phone to this user. You can leave voice mail to extension 7xxx and check voice message by dialing into voice mail pilot number 395 and then mail box xxx.
UC320 doesn't support Auto Attendant embedded within voice mail feature.
Configure Multicast Paging on the Cisco IP Phone 7800 Series or 8800 Series Multiplatform Phone
The Cisco IP Phone 7800 and 8800 Series Multiplatform Phones provide voice communication over an Internet Protocol (IP) network...
Add Call Park on a Cisco 7800 or 8800 Series Multiplatform Phone Key Expansion Module
Call park allows the user of the phone to put an incoming call on hold so that the call can be retrieved on another phone. A call is park...