So I got my new HTC One X from AT&T and the I could not set google voice as my voice mail.

Issue :

When Google Voice is selected at the voicemail you get an error “Call settings error: Failed to retrieve and save current forwarding number settings.”
And even if you select “Yes” on the save settings it reverts back to carrier voicemail.

Solution:

Open Google Voice in your browser (computer)
Select Settings
Select Activate Phone (or Deactivate Phone first and then Activate)
You will be prompted with a # to key in on your phone *004*yourgooglevoicePhoneNumber#
Hit Call or Dial and it’s up and running

Tested and Validated by ME