正好24小时。全程自助，方便高效。感谢 @sincoe 清晰明了的攻略以及耐心解答。 
收到的邮件里反复提醒如下： 
Google Voice is not a mobile phone service provider, so you must set up a new mobile phone service plan (with your existing carrier or a new carrier) and request a new number. 
You will need to add this new number to your Google Voice account as a forwarding phone in order to receive calls on your mobile phone. 
我理解是因为Google Voice是没有套餐只有免费...