Google Voice may have its own mobile webapp now, but it doesn’t provide notifications on your phone when you get new SMS messages. Free web service GVMax solves this problem by putting full SMS support into email and IM on your smartphone.

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GVMax is an open source web service that allows you to send and receive SMS messages through either email or GTalk, as well as get push notifications of new SMS messages on your iPhone without keeping your computer on at home.

Full article here:
GVMax Forwards Google Voice Notifications to Your Phone (Lifehacker)
Related:
GVMax – Most comprehensive Google Voice Notification system [Tutorial] (TruVoIPBuzz)
GVMax makes Google Voice and iPhone Play Nice (Brain Dungeon)