Very simple cross-platform app that just makes a call

I would like to have developed a very simple app with the logo and graphics of my client that just makes a phone to a predefined number. Once the user opens the app it should call right away to a helpline number.
It would be great if the number could be read from a WS but the app shouldn't require Internet. This number hasn't changed for 10 years now and its not supposed to but it would be could to have a solution in case the number changes. Any ideas are welcome on this matter.

Also this number is already free to call by any number in Greece. So we will have to add an extra control to check the geographical location of the device. If for example someone opens the app and calls this greek number but he is based in Italy he should be able to see something like: "This call you are trying to make may not be free, do you want to proceed?" .

When the call ends the app should stop functioning somehow. Perhaps with an only option of an exit button.

This app should be built for Android, iOS, BlackBerry and Windows Phone.