The firmware (iPhone operating system) is outdated by at least a year. The app requires a more current firmware version. You'll be able to install app after you connect to iTunes on your computer and update. You'll need to follow specific directions if your phone is unlocked. Let me know if you need a link.
Are you 100% certain it's unlocked? Is it a Vodaphone iPhone or was it sold through another carrier? Do you have Cydia on the iPhone? If so, open it up, take a screenshot of the homescreen and post it here. I need to see if you have any 4x SHSHs backed up.