Hi Phillip,

Just to tac on to the great info from Brad (+5 points here Brad)

A separate server is not required (which is good news) :)

This option has been available since 4.0 (4.0.5 at least) Have a look;

Class of Service Messages Settings

Class of service messages settings specify the maximum length of messages, whether subscribers can send messages to public distribution lists, how deleted messages are handled, and whether subscribers can use the live reply feature.

When live reply is enabled, subscribers listening to messages by phone can reply to a subscriber message by pressing 4-4 to have Cisco Unity call the subscriber. (Subscribers who use Optional Conversation 1 press 8-8 for live reply.) For single-server installations, Cisco Unity dials the extension of the subscriber who left the message only when the Transfer Incoming Calls to Subscriber's Phone setting for the subscriber who left the message is set to ring an extension or another number. (The Transfer Incoming Calls to Subscriber's Phone field is on the Subscribers > Subscribers > Call Transfer page in the Cisco Unity Administrator.) In addition, the call transfer settings for the subscriber who left the message dictate what Cisco Unity does when the subscriber phone is busy, and whether Cisco Unity screens the call.

Subscribers Can Reply to Messages from Other Subscribers by Calling Them Check this check box to allow subscribers to use the live reply feature. When this check box is checked, subscribers can press 4-4 after listening to a subscriber message, and Cisco Unity will call the subscriber who left the message. (Subscribers who use Optional Conversation 1 can press 8-8 to use live reply.)

When this check box is unchecked, live reply is disabled in the subscriber phone conversation.

Subscribers > Class of Service > Messages Page

Hope this helps!

Rob