Can I use my iPod mini as a phone?

Not in the sense that you are thinking. You can make FaceTime audio calls or use Skype, but the iPad can not be used as a phone.

Yes, you need to get a voip app ... Voice Over Internet Protocol. search voip in app store.

see for example:

https://itunes.apple.com/us/app/wetalk-cheap-calls-voip-rates/id455447012?mt=8

Google hangouts has free voice call. I tried it on my iPhone 4. poor quality.

I'm using Freedompop.com voip app on my iPhone 4.

R