iPhone 3GS silent when in cradle?

Don't know if I’m being stupid here, but I have looked through the forums and can't find anybody else with the same problem… I have two charging cradles, one here and one in work, where the phone spends much of its time.

Trouble is, when it's in either, it goes silent – ie I can’t hear when I get a text or an alarm of whatever. It DOES ring though. This is very, very annoying. I've gone through all the settings but, unless I’ve missed anything, there isn't any way of fixing the problem.

Is this irritating behaviour by design?

Thanks :D