The Online Family answers:

If you already have Internet access, then in most cases you can use voice chat programs at no cost by way of free instant message tools -- like Yahoo Messenger, AIM ,or Skype -- that also allow you to "talk" to friends in addition to text chatting. On the other hand, Internet telephony services, such as Vonage or AT&T's CallVantage, require a monthly subscription and special equipment to use the Internet to place traditional phone calls. Still, watch out for unexpected fees. For instance, with Skype you can also dial regular phones from the "free" online program, but you'll be charged about one cent per minute in this case.