Apple opposes the use of cellular networks for VoIP calls, thereby ruling out the use of 3G and 4G networks, but leaves the door open for Wi-Fi. So, you can use your iPod Touch in any Wi-Fi hotspot or around a Wi-Fi router to make unlimited local and international calls, for free or very cheap. However, WiFi is quite limited. You will not be able to communicate while on the go unless you are in a hotspot, which is far from being everywhere. Using mobile data would make the iPod a complete communication tool.
One way is to use a VoIP app for smartphones that is compatible (designed for) Apple's iPod Touch. While there are numerous apps out there for online communication, only a handful are compatible with the iPod Touch. Here are some apps you can try:
Skype: The oldest app out there. It comes with a great list of features and allows voice calls and instant messaging for free online. It also allows you to make calls to international destinations for cheap.