VoIP services have already put a lot of pressure on landline services. Now, VoIP is putting the pressure on mobile carriers as well. More and more mobile devices like Apple’s “iPhone” are able to use software and networks a mobile VoIP client needs to operate. However, mobile carriers have set a few ground rules as to what is allowed. One example is that AT&T forbids the iPhone to run VoIP over its cellular network. Doing this limits its use to Wi-Fi.
Wireless VoIP services vary widely in how they work. Some services, including “Jajah”, can be used with basic cell phones and it is required that users call a local number that will then connects them to the service that then routes a phone call overseas or anywhere in the world. Others are more involved, requiring that the user must download software onto a cell phone that has internet connectivity.
In some cases, calls must be placed through a Wi-Fi network connection. What all of the services have in common is that each one helps the wireless users circumvent at least some of the charges associated with long-distance wireless calls.
I hope all phone will eventually allow you to use VoIP.