While many companies are looking to compete head-to-head with Skype and Vonage, a few are using VoIP to address other aspects of a similar problem.

Such is the case with a new service from Swedish startup REBtel. The service lets cell phone users place and receive free or low-cost international phone calls from around the world, albeit with a few headaches.

REBtel offers two services: REBout and REBin. REBout allows you to dial a local number to call friends across 35 countries. You simply enter your friend's phone number either at the REBtel site or on your cell phone, and REBtel returns a new "local" phone number for both you and your friend. Your friend can then call that number with you paying the local rates, a service charge and $1 per week for all phone calls you make using the REBtel service. The service charge is country dependent and ranges from 2 cents to 26 cents per minute. Calls from Israel to the United States, for example, run 2 cents per minute.

You can avoid paying the service charge by getting your friends to sign up for the REBin service. This way, when friends call you on your REBtel number, you can call them back back on their REBtel numbers and speak for the cost of the local call. Similarly, if you call a friend, you can hang on and have that person call you back on a REBtel number, again avoiding the service charge. The REBin service works in 30 countries. The voice quality on the service was fine on one test call between Jerusalem and Boston, but far worse on subsequent calls between from Jerusalem to New Jersey and New York.

Because REBtel phone numbers are specific to each caller's telephone, the service is far more complicated to use than is necessary. And users can't call REBtel numbers from extensions in companies that block caller ID. If the company issues a single phone number for all extensions, the service may work, but only if the REBtel number is linked to that overall number.

What's more, you can't give out a REBtel phone number as a general mobile number. The REBtel number is only available to the user of the linked number. This means that users you don't yet know can't call you on your REBtel number. And the process of configuring REBtel numbers can be time consuming. Only some of this pain is eliminated by the REBtel Outlook add-in, which automatically enters Outlook contacts into the REBtel service.