Iphone – is it possible to open Settings App using openURL

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.

I think the least intrusive way of doing this is as follows:

1. Check if the user-agent is that of an iPhone/iPod Touch
2. Check for an appInstalled cookie
3. If the cookie exists and is set to true, set window.location to your-uri:// (or do the redirect server side)
4. If the cookie doesn't exist, open a "Did you know Your Site Name has an iPhone application?" modal with a "Yep, I've already got it", "Nope, but I'd love to try it", and "Leave me alone" button.
   1. The "Yep" button sets the cookie to true and redirects to your-uri://
   2. The "Nope" button redirects to "http://itunes.com/apps/yourappname" which will open the App Store on the device
   3. The "Leave me alone" button sets the cookie to false and closes the modal

The other option I've played with but found a little clunky was to do the following in Javascript:

setTimeout(function() {
  window.location = "http://itunes.com/apps/yourappname";
}, 25);

// If "custom-uri://" is registered the app will launch immediately and your
// timer won't fire. If it's not set, you'll get an ugly "Cannot Open Page"
// dialogue prior to the App Store application launching
window.location = "custom-uri://";