A few years ago, some companies like Stoke and Bridgeport Networks emerged with a noble vision: Create a super phone network technology that lets you move seamlessly back and forth from your home WiFi connection to the cellular network outside. That way, you’d use the strongest and least expensive network available to you at the time.

While the vision has partly been realized — there are phone plans that let you do this — the sector has been encumbered by consumers unable to figure out the confusing technology, and carriers who who aren’t aggressively pushing or explaining it very well.

Still, Stoke, one of the players leading the charge, is hanging on to its mission, and has raised $5 million more from DOCOMO Capital, a firm affiliated with one Japan’s largest network carrier, and Mobile Internet Capital. This extends the company’s fourth round of funding to $20 million (earlier this year, a number of other investors, including Kleiner Perkins and Sequoia Capital, invested $15 million).

Soon after Stoke launched two years ago, the company’s VP of marketing Keith Higgins was telling me that the carriers were stalling in upgrading their technology, and he sounded frustrated. No wonder that he soon left the company.

Basically, WiFi/WAN roaming never quite took off quite the way some people expected, and it’s unclear whether the it will ever be substantial. But Stoke is still around, and striking deals. As people use mobile devices for more data-intensive applications, i.e, YouTube, Slingbox, it make sense for operators to want to let, or even push, their customers onto WiFi or femtocell technology in order to conserve costs. However, at the same time, they don’t want to lose their customers to other providers, and so want to control the process. Another problem is that femtocell technology is more costly to manage than some initially thought. However, the fixed-wireless convergence movement does continue: Vikash Varma, chief executive officer at Stoke, said earlier this year that 2009 would be the year when carriers moved “to reduce the cost per bit of mobile data and off-load internet bound traffic as quickly as possible to allow them to support the surge in demand.”

Stoke has now raised a total of $70 million. Other investors include DAG Ventures, Integral Capital Partners, Net One Systems, Pilot House Ventures and Reliance Technology Ventures.

The company said the most recent funding will be used to support Stoke’s commercial deployments in Asia Pacific, of which Japan is the most advanced, and the imminent launch of new 3G “offload” capabilities for the Western European market. In June Stoke announced it had been selected to supply NTT DOCOMO with its advanced mobile broadband gateways for commercial deployment.