if you're making a call from your application and you want the user back into the application after the end of the call then use telprompt: instead of tel: like below
-(IBAction)MakePhoneCall:(id)sender
{
NSURL *URL = [NSURL URLWithString:[NSString stringWithFormat:@"telprompt://%@",detail.tel]];
[[UIApplication sharedApplication] openURL:URL];
}