I've an app, that permits me to call to my contacts. I use the following code to do that:
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:numberVar]];
You want to use IPC (inter process communication), which is a bit odd in iOS apps: You need to register a protocol handler in your app and place a link in the HTML page you're displaying with
openURL. E.g. your link could start with something like "myapp://" instead of "http://".