Launch an app from within another (iPhone)

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 found that it's easy to write an app that can open another app.

Let's assume that we have two apps called FirstApp and SecondApp. When we open the FirstApp, we want to be able to open the SecondApp by clicking a button. The solution to do this is:

1. In SecondApp

   Go to the plist file of SecondApp and you need to add a URL Schemes with a string iOSDevTips(of course you can write another string.it's up to you).

2. In FirstApp

Create a button with the below action:

- (void)buttonPressed:(UIButton *)button{  NSString *customURL = @"iOSDevTips://";  if ([[UIApplication sharedApplication] canOpenURL:[NSURL URLWithString:customURL]])  {    [[UIApplication sharedApplication] openURL:[NSURL URLWithString:customURL]];  }  else  {    UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"URL error"                              message:[NSString stringWithFormat:@"No custom URL defined for %@", customURL]                              delegate:self cancelButtonTitle:@"Ok"                               otherButtonTitles:nil];    [alert show];  }}

That's it. Now when you can click the button in the FirstApp it should open the SecondApp.

The lee answer is absolutely correct for iOS prior to 8.

In iOS 9+ you must whitelist any URL schemes your App wants to query in Info.plist under the LSApplicationQueriesSchemes key (an array of strings):