[dba-Tech] VoIP with a smartphone?

An article in PC Today says the following:

"VoIP ... services, such as Skype ... are making it easier for business 
professionals to make phone calls over the Internet and avoid paying steep 
fees for international calls."

Okay, does this mean I can use a smart phone to use VoIP? It certainly 
sounds like it. How do I do that? What about your basic cell phone?

I haven't tried Skype. We have a magicjack at home though, which certainly 
isn't wireless -- would like to learn more about this.

Susan H.