There are actually several apps for making (and receiving) calls on a mobile, whether on Wi-Fi or using the 3G network. Viber is a popular Skype alternative that I know some use. If you use an SIP provider, Android 4 has support for this built in.
I've no mobile signal at home, so I use the SIP provider Blueface, which also lets me make calls through it, plus my mobile rings if someone calls my Blueface number, even from a landline telephone. In the past, I had a corded SIP phone that connected to my router, so nice being able to do away with that with my mobile. Another advantage with using Blueface (or even Skype) is that they work over 3G, so the calls are very cheap or free if I call another person by SIP or Skype.