Iphone 4- bought an unlimited data plan but it says I need Wi-Fi for facetime?

I should just be able to use the 3G right? Why would I need Wi-Fi? That's the reason we bought an unlimited data plan; so that we wouldn't need Wi-Fi. The internet, however, works just fine, other than being a little bit slow, but it connects to the internet and app store just fine without Wi-Fi. It only happens when I'm trying to run apps like snapchat or trying to use facetime. Why is this?

Additional information- I have cellular set to on in my settings. I have Wi-Fi turned off in my settings. I just got this phone yesterday and I've never had an iphone before so is there something I didn't set up?

Added (1). My family and I didn't spend any money on it. It was free as an upgrade. And they surprised me with it. So next time don't make assumptions. And as I said before, I've never had an Iphone before so how was I supposed to know. If I had paid for it, I would have done research.

Sorry but iphones 4 + 4s don't allow it without Wi-Fi. Do some research before you spend your money on an iphone, but the new one and androids allow it

In the current smartphone market, service providers are practically milking people off their dough in the name of data plans. Once you get a smartphone you will undoubtedly be spending around $100 to $130 on a data plan that you will probably not be using anyway.

More you read here: http://mobilerookie.com/using-iphone-without-data-plans/