Let's say you have to dial the conference number 1-800-828-2992 with a pass-code 123456. Use the following to invoke the phone app and insert a pause and the let it dial the passcode.

NSString *conference = @”18008282992″;
NSString *passcode = @”123456″;

NSString *telUrl = [NSString stringWithFormat:@”tel://%@,%@”, conference, passcode];
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:telUrl]];

The key here is to understand that , can be used to insert a pause and shown above before dialing the pass-code.