Google Voice-specific Settings
In this dialog, Google's proprietary SIP header X-Google-Pbx-Trunk-Secret-Key must be set. This unique header will be generated when creating the anynode SIP trunk in Google Voice.
More information can be found at the end of the Add a SIP Trunk, and the Showing the SIP Trunk Secret Key chapter. The header is located in the SIP User Agent of the corresponding Google Voice node object.
Screenshot: anynode frontend showing the Google Voice dialog for setting up the secret key for the X-Google-Pbx-Trunk-Secret-Key
SIP header.