Most WiFI VoIP phones don't have internet browser which makes it impossible to log into public hotspots that require web page based authentication.
However, if you spoof the phone's MAC ID with another computer and use it to connect you can then use the VoIP phone. It's an elegant solution. I don't typically carry my laptop around but this method works even with palm sized devices like Nokia N770. The tutorial walks you through Windows and Linux MAC ID spoofing. The trick can be used for other devices beyond WiFi phones.