Vodafone hass developed a system that allows mobile phone customers to call and exchange presence information with VoIP users.

The application, which is just a prototype right now, is called Starfish. It makes use of two switching systems: the circuit-switched technology used in GSM networks and the packet-switched VoIP technology, said Jochen Hertle, director of new business development at Vodafone.

Calls initiated on mobile phones are transmitted over the airwaves via traditional circuit-switching technology. A gateway at the mobile switching station coverts the call into IP and forwards the packets to the PC user via their VoIP service.

Conversely, PC users can initiate a VoIP call, which is converted to circuit-switched signal at the media gateway and transmitted over the airwaves to mobile phone customers.

In addition, Starfish uses GPRS technology to provide the presence function featured in Internet messaging services such as AIM, Skype and Yahoo, according to Hertle.

Other companies have tried to achieve the same results with an all-VoIP approach, putting client software on smartphones, and using mobile data connections to carry the voice traffic.

In contrast, Vodafone's hybrid approach with Starfish has a couple of benefits, said Jan Holzberg, new business product manager. "First of all, the voice quality is excellent because we use GSM over the airwaves," he said. "The quality of VoIP calls over the air is nowhere as good largely because of low data speeds, and even when new higher-speed transmission rates become available, circuit-switched voice quality will remain noticeably better."

Second, Starfish works on all GSM networks, which are ubiquitous, unlike all-IP mobile networks, which are available mostly in large, urban areas, according to Holzberg.

Starfish generates call fees, however. Mobile phone customers must pay to initiate a call as do PC users connecting to a mobile phone network.

But Hertle pointed out that VoIP calls on mobile devices also carry a fee; users must pay a mobile data charge. "There will always be a cost to communicating on a mobile phone, whether voice or data," he said. "The issue will be which is cheaper."