One word that abounds more than any other when searching for a VoIP provider is the word "free." "Free" has always been used in advertising and marketing--because who doesn't want something for nothing? As far as VoIP service is concerned, though, "free" usually refers only to PC-to-PC calls.
PC-to-PC does not necessarily refer only to computer-to-computer. A more accurate term might be IP address-to-IP address, because VoIP telephones look and act just like regular telephones except that they work over your internet connection. While it is possible to make calls from your PC with a VoIP headset, or a microphone and a set of speakers, most VoIP customers use VoIP telephones.
While absolutely free VoIP service (meaning free calls to other VoIP users and non-VoIP users) does exist, such services are usually limited in some way. These limitations can include short call times, geographical restrictions, or the downloading of ad-supported software. The only absolutely free and unrestricted calling provided by most VoIP carriers is VoIP-to-VoIP.
When it comes to choosing the best VoIP provider, cost is generally a secondary concern, because VoIP service will almost assuredly save you money on long distance and international calling. Unless you make lots of VoIP-to-VoIP calls, your free calling may not account for much. Your major concerns should be the quality and reliability of service, as well as the degree of technical support your VoIP carrier will provide.