Hello, a PPTP VPN is cross platform (ad I will admit it is not the best but it gets the job done). It works natively with iOS and Android (WinMo support as well). The advantage is that you dont need to root or Jalbreak the device to use it.
The problem is however many mobile phone providers filter out the GRE Ports. In my case, I actually had to get a dedicated IP address assigned to my phone