Account Inactivity
Google may reclaim your Google Voice number (if you have one) if you have not placed or received calls, sent or received text messages for a period of 6 months. We will not reclaim numbers that have been ported in to Google Voice or made permanent.
https://www.google.com/intl/en-us/googlevoice/program-policies.html
最后一句话是“Google不会收回已转携至Google语音服务或申请了永久使用权的号码”