To go back to original app you can use telprompt:// instead of tel:// – The tell prompt will prompt the user first, but when the call is finished it will go back to your app:
NSString *phoneNumber = [@"telprompt://" stringByAppendingString:mymobileNO.titleLabel.text];
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:phoneNumber]];