Say you have to dial the conference number 1-800-828-2992 with a passcode 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 passcode.