I have found the below information
Unsupported Phones
cBarge and Privacy features cannot be configured for single-button phones and
phones that do not support shared octal-line directory numbers (DNs). Phones that
do not support shared octal-line DNs are listed below:
................
All Cisco SPA500 Series Phones (Models SPA501G, SPA525G, SPA525G2,
and SPA50x)
In Page 402 of
http://www.cisco.com/en/US/docs/net_mgmt/cisco_configuration_assistant/version3_2/administration_guide/cca_sbcs_admin_guide.pdf
This to me looks like it is against the SPA525G Guide !!!!!!!!!!!!!!!