The iPad is not a cellphone and cannot make calls natively. You would need to use one of the Voice-over-IP services such as the Tango service you mentioned. I haven't used Tango so I cannot comment on how well it works. The reviews in the iTunes Store are quite mixed; people seem to either love it or hate it.
Whatsapp works only on iPhones, not on the iPad nor iPod touch.
No you can not make calls. WhatsApp only works on iPhones. I'm not sure about Tango. It can use Wi-Fi so it might work.
Skype is an app that allows you to make video calls to any other internet connected device, not necessarily an Apple device.
You are no doubt aware you can use FaceTime to make video calls to iOS and OS X devices.