Mobile VoIP is becoming a good commodity for wireless carriers
In layman's terms, mobile VoIP provides people to send and receive phone calls using some third party mobile VoIP service provider (e.g. Skype) over internet connection. Mobile VoIP service provider provide this service free of cost (or very nominal charges). This is different from standard cellular service because call is not placed on your wireless carrier's network and the user is not charged any carrier fees. This service is particularly useful for making long distance calls.
Mobile VoIP is an extension Voice over IP (VoIP) on mobile device where a mobile handset is integrated into a VoIP network. The mobile handset actually continues to operate as a GSM based device as provided by wireless carriers. But with Mobile VoIP, these Mobile device also can be controlled by a SIP server which can now provide SIP based services. You can imagine voice over Wi-Fi where mobile device can potentially make free voice calls but it is only available within the coverage area of a single Wi-Fi access point.
This ability of making long distance calls without using any cellular minutes seems very attractive and valuable for the mobile users. The charges associated with Mobile VoIP tend to be very low as compared to the standard cellular minutes. But there is other side of the story that needs to be taken into account and that is the cellular companies. How would they run their business if the main source of their revenue that is the use of voice network is replaced by mobile VoIP providers. That was exactly the reason why some wireless carriers blocked the use of Skype over their networks initially in last decade.
Although blocking any useful utility wasn't a solution but at that time only small amount of mobile users were using the data services of their cellular networks. A large source of income was coming from voice users. People deemed it valuable to make long distance calls through mobile VoIP service through Wi-Fi in home, office or public locations.
But as the data service subscribers are increasing and becoming part of the must-have service, the mindset of the cellular companies also changed. And the reason is that alternate revenue stream of data service is created for cellular companies along with the main revenue stream of voice services. These companies are recognizing need for mobile VoIP and mobile application development.