I know this is off topic, but I've got a feeling that GV is on the cutting board. Google just isn't paying any attention to it, and after their more recent cuts...
I went ahead and gave it up. It was painful, but it was ripping off a band-aid.
I'm using the "Google Voice Phone" and "Google Voice SMS" extensions that were updated for iOS 6 by Andrew Chilvers. The SMS extension works perfectly and the phone extension works but every once-in-a-while I get the "Error: Returned Null" problem that I have to fix by logging out and back in on the GV App and mobile Safari.
I don't know if these are the "extensions" the OP meant or not.
I know this is off topic, but I've got a feeling that GV is on the cutting board. Google just isn't paying any attention to it, and after their more recent cuts...