Did you ever get a reply to this question? I cannot find anything about changing the rings with Google Voice either. However, some mobile providers will allow that, but sometimes you have to have them directly set it on their computer if there is no option on the phone itself. But according to ObiHai Technologies, on May 1, 2014, there is a change being made with Google Voice and they will no longer support the XMPP protocol. I'm not sure of what that all means except those of us who use Google Voice with the ObiTalk adapter for free phone service will no longer be able to do so. I understand Obihai is working to get some alternatives for the users - here is the link
https://www.facebook.com/obihaitechnology, but that apparently won't solve your problem. Have you tried entering a search on Google itself (not Google Voice)? Google Voice is a nifty tool, but the support is JUST TERRIBLE, as happens with such a big monstrosity as Google. So, you might want to look into what Obi has to offer.
Take care,
Jerry in Michigan, USA