What happened to my Call Waiting?

I have the brand new white iPhone 4 with the latest updates.

Problem: Whenever I was in the middle of a call and got an incoming call, it would beep and give me the option to take the incoming call. Now the last few days whenever I get a call in the midst of another, it goes straight to voicemail and only notifies me then -- it no longer gives me the option - like Call Waiting - to take the new call and as a result I've missed and upset a lot of contacts.

Is there a setting I messed up here? How can I get Call Waiting back?

It has happen to me a couple of times and its not the phone chalk it up to the carrier.

here is the phone call setting for iphone:iPhone: Understanding phone features
Pls check the settings...