About and Configuring Whisper Intercom in Cisco CME 7.1 and the UC520

When a phone user dials a whisper intercom line, the called phone automatically answers using speakerphone mode, providing a one-way voice path from the caller to the called party, regardless of whether the called party is busy or idle.
 
Whisper intercom allows a user to call to a busy extension. The calling party can only be heard by the recipient via a one way audio path.  A phone that is on a call  is notified of  a whisper page by displaying the extension and name of the party initiating the whisper page and Cisco Unified CME plays a zipzip tone before the called party hears the initiator's voice. To speak back to the initiator the called party must depress the intercom line button. The lamp for intercom buttons are colored amber to indicate one-way audio for whisper intercom and green to indicate two-way audio for standard intercom. 

 Whisper Intercom is configured individually for each DN that requires the whisper feature. A whisper intercom directory number can place calls only to another whisper intercom directory number. Calls between a whisper intercom directory number and a standard directory number or intercom directory number are rejected with a busy tone.
This feature is supported in Cisco Unified CME 7.1 and higher.  
A good configuration example is in the CME Administrators guide located here:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmeinter.html#wp1019638