I wrote about the vast array of ways to bypass international tolls in my Internet Telephony column a while back. Now there is an interesting web site, LowCostMob.com, that gives a listing of the services available and technical explanations of how they work.
If you go to the “contact us” link on the website you can type in “user feedback” with mini-reviews of the services. I presume that over time the database of user comments will become an additional helpful resource on the site.
All these services work to make calls to international destinations cheaper, but if you actually travel abroad you still have to pay exorbitant roaming charges for using the cellular network. The benefit of dual-mode (Wi-Fi plus cellular) phones is that with some of them you can use the Wi-Fi connection to make VoIP calls and completely bypass the cellular network, avoiding international roaming charges. Not all the listed services support this feature, and not all dual mode phones do either.