Mobile VoIP providers use your cell phone as a SIP trunking station. If you're already paying for data through your cellphone provider and you're already paying for VoIP from your VoIP service provider, mobile VoIP means you don't have to pay anything extra. Just download the softphone from your VoIP provider.
Software-based mobile VoIP providers rarely, if ever, charge for calls within their network. Usually, mobile VoIP providers have a debit-based system: you deposit a certain amount of money to your account and they charge that account for the off-network calls you make. Remember, calls within the network are free.
Since software-based mobile VoIP provider services can be downloaded, they run on computers and smartphones. As 3G coverage improves, 4G connections bring faster connection speeds, and Wi-Fi hotspots increase, mobile VoIP software will become more and more widely-used.
Mobile VoIP Apps
VoIP applications for your cellphone are becoming more common. VoIP providers may include a free app for download with your phone service, or providers may be entirely based around softphones and phone apps.
Skype, among others, has a free app for mobile phones and tablet computers. You can make calls to other Skype users for free (except for the data usage on your phone) or you can call other phones with Skype credit. Many other mobile VoIP providers have the same terms.
You need a data connection to use apps like Skype. You can either get data from Wi-Fi, or from your cellular plan's data. If you need a valid United States cell phone plan, check out Wirefly's plan comparison tool.
Local Access Mobile VoIP
Some mobile VoIP providers like Rebtel use local access numbers for the both your origin and destination you want to call, allowing cheaper, local minutes for both parties.
While Rebtel's main feature is the array of phone apps, the same service has international phone calls from its website. You sign up and give Rebtel your phone number and the number you want to call. Rebtel gives you a unique local number for the contact, which you can call from the phone you signed up with. You pay for the local call to whatever phone service you currently have, and a small per-minute rate to Rebtel for the international part of the call.