There is a way to block unwanted calls from your iPhone, Go to your contact list and select "edit google voice settings". Next select "when this contact calls you". It will then give you three options to choose from. The first one is "send the caller directly to voicemail" Your phone won't ring but they will be able to leave a message. The second choice is "mark the caller as spam". Your phone will not ring and if they choose to leave a message, it will go directly to spam. The third choice is "block the caller". The caller will be met with a message stating that "this number is not in service". and they will appear on you missed list as "Blocked"
on the new iphone it's so easy. just go to your call registry, the place where you can see all the calls you made, missed, and received. Then click the little "i" icon on the right next to the number. the next screen that it brings up will ask if you would like to add, delete...yadayadayada...scroll all the way down and there is a lonely option to block the caller. easy as pie