请问ATT android手机到期后转prepaid card哪家好？

没用过prepaid，看似有按月或三月，或按钱的卡？如何充值？
如果上网那个plan好？如果不上网呢？

are these true?
http://www.pcmag.com/article2/0,2817,2414494,00.asp

looks like ATT has $15/month, $25/3months and $100/year prepaid card, and after the time expires the moeny is gone and user still have 6 month for a/c holding, after that a/c closed.
one can buy $0 SIM card for $5 on Amazon, or card with money than the %5 is saved.
http://www.att.com/shop/wireless/plans/prepaidplans.html
http://www.pcmag.com/article2/0,2817,2414495,00.asp

 如果用ATTphone改为它的prepaid，原号码还能用吗？