This New York Times article
has a useful survey of internet phone services that can save you money whether you make calls from cell phones or from your landline. Two free internet services mentioned in the article are Google Voice and FreePhone2Phone.com.
I've been using Google Voice, and I've been happy with it. It allowed me to register a free local phone number, and it allows me to make free domestic phone calls. I can have incoming calls forwarded to my landline or cell, or I can have Google Voice be an answering machine. Messages are not only recorded but converted to text and sent to my gmail account.
I just learned about FreePhone2Phone.com from the article. You find a local number on its website, and use that number to call long distance. When their system answers, you're prompted to dial your destination number. It's suppose to play 1 or 2 ten-second ads before it lets you dial out. Then they give you 10 free minutes. I just tried it, and it let me dial immediately without any ads.
I'm a little worried about whether FreePhone2Phone can last. I used to use a similar service 10 years ago (I think it was called PhoneHog), and it didn't last. It couldn't make enough money from the advertising.