Skype
Skype is one of the largest free VoIP networks. You can connect directly from your computer
Free VoIP
to millions of worldwide users at any time of the day or night. It is simply to join and simple to use. I suggested that one of my students join Skype because his wife was making daily long distance calls from Southern California to Mexico and the phone bill was getting out of control. After he joined Skype the long distance charges dropped to zero and everyone in the family was happy because his wife could now talk as long and as often as she wished. There are even Skype mobile phones available now that can connect you to Skype at Wi-Fi hotspots like Starbucks, which are now everywhere including many overseas. So mobile Skype phone could be a cheap and convenient way of keeping in touch when you are traveling over seas. Skype even has an inexpensive option for connecting to landlines for those who are not signed up with Skype.
Google Voice
Google is developing a new free VoIP network service where you can select a number and have it forwarded to all your existing numbers. You can receive and make web based calls anywhere is the U.S. (including Alaska and Hawaii) free of charge. They even have low cost overseas options. The service is in Beta and by invite only (invites are still available for the asking) but someday should be open to everyone.
Mobile Phone Providers
Most mobile phone providers are now developing and offering VoIP options on their mobile phones and you will be able to make free calls from hotspots or over data links through the provider. This again would be very helpful for the overseas travelers.
Free VOIP Gateway
There are even software options that can be implemented for developing a PC software VoIP gateway solution. These are Linux applications based on the free Open Source initiatives so you will find a very active community of developers and users to help if you are wanting to have a VoIP gateway to service your business with all the business features that you would need.
Since all calls are now converted to digital and sent over switched network systems it will not be too long when all voice and data merge. The VoIP calls now are such high quality that you cannot tell if you are on a voice or data network making your free VoIP calls.
The VoIP Adapter website has addition information that you might find helpful.