What about the CAPTCHA challenge? It's possible that the number of requests being sent to Google on your behalf to fetch your contacts is triggering a CAPTCHA challenge from them. If that's the problem, please submit this form using your Google Voice username and password:
https://www.google.com/accounts/DisplayUnlockCaptchaIf that's not the problem, try deleting your contacts from VoiceCentral and trying again. To clear your contacts in VoiceCentral, go to the
Keypad tab, enter
##335*266#, and tap
Call. Then try to sync again.