Is FaceTime free of Charge when conecting to other iPhone users worldwide?

Yes it is a free service. The only thing is that it uses your network (4G, 3G or WiFi) and depending on how you are connected and what service is on offer, you may pay for data usage. Apart from this, it is free.

I should also add that it works across iPhones, iPads and Mac computers. So you could use your iPhone to call an iPad or a Mac computer.