According to the help page:
"To access custom Google Voice settings for a particular contact:
1. Sign in to your Google Voice account.
2. Search for your contact's name in the search field at the top of the window, and when it appears, click the contact's name.
3. On the contact card that appears, select edit settings. From there you can edit your contact's Google Voice specific settings."
I tried it many times, on many different machines, using different browsers; clicking "edit settings" did nothing to me, it's a broken function apparently. I wish they at least tested it before forcing everyone using google contacts.