Here is a list of the most prominent VoIP services providers of this type:
1. VonageVonage tops my list because it delivers the goods. The price is not the lowest, but not the highest as well. Call quality is great, as well as the service and all it entails, including the features and customer service. The adapter is provided for around $40, refundable on returning it. 14-day trial.
2. LingoFor $21.95 a month, Lingo offers unlimited calling to the US, Canada and 22 other countries. There are around 25 features with the servoce, and a 30-day money-back guarantee. Equipment comes free.
3. Voip.comVoip.com offers VoIP service at a price lower than most of the competition, $16.95 per month. Among the strong points are a huge amount of features, the softphone that ships with the service, and a second line and voicemail for your mobile phone. They provide a Grandstream ATA, shipped free. A 30-day money back guarantee allows you to try the service.
4. BroadVoiceIf you make a lot of international calls, Broadvoice is the company to subscribe to. It offers service plans that are specifically tailored to users that call mostly outside the US, without however sacrificing local calls. Another interesting thing with Broadvoice is its BYOD (Bring Your Own Device) scheme, whereby you can bring along your own device and sign up with a fee starting at $5.95 a month. The service works with most SIP-capable devices. The money back guarantee period is 30 days.
5. Packet8Packet8 is an important player in the VoIP market as it has a series of services, including a hosted PBX and a mobile VoIP service called MobileTalk. The price is the standard $24.95, and with $5 more you can call unlimited to 40 countries. It has an activation and a termination fee and 30 days trial period. They provide an IP phone instead of an ATA. You can use their softphone SoftTalk for free.
6. ViaTalkWhat draws most people to ViaTalk is the price, even though they have to commit themselves for a year to get the cheapest price. ViaTalk also stands out with the long list of features it has. It also allows users to use their own devices (BYOD), which saves them from paying for activation and shipping. It has a money back guarantee of 14 days.
Update: AT&T has closed subscription to the CallVantage service, but existing users can still use it.