problem using tel: URL to initiate a call

I'm trying to initiate a call from within an iPhone app.

This related code works and opens Safari as expected:

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"http://www.apple.com"]];

But, when I replace the http URL with a tel URL the resulting code does not invoke the phone app:

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"tel:3035551212"]];

No exceptions or alerts are generated (in the simulator or on a device).

Any idea what the problem might be with my invocation?

Thanks.