Iphone – phone restore the state of app after [[UIApplication sharedApplication] openURL:[NSURL URLWithString:call_url]];

As Kevin points out, URL Schemes are the only way to communicate between apps. So, no, it's not possible to launch arbitrary apps.

But it is possible to launch any app that registers a URL Scheme, whether it's Apple's, yours, or another developer's. The docs are here:

Defining a Custom URL Scheme for Your App

As for launching the phone, looks like your tel: link needs to have least three digits before the phone will launch. So you can't just drop into the app without dialing a number.

As others have answered, your app likely won't be approved if this is all that your app is going to do, but to answer your question:

Your problem is that the app is multitasking on iOS 4, and -[UIApplicationDelegate didFinishLaunchingWithOptions:] doesn't get called again when the app comes back into focus from the background. (More on how the app delegate methods work with multitasking here.)

If this is all your app is going to do, then the easiest fix is to simply opt out of multitasking.

If you don't opt out, then you need to put that same 'openURL' logic in one of the app delegate methods that gets called when multitasking is used, for instance -[UIApplicationDelegate applicationDidBecomeActive:].