Tesco has launched its new Talk Wifi service (via Pocket Picks), offering users of a few Nokia S60 devices a downloadable application that lets them make cheap calls over WiFi, to go alongside its low-cost MVNO and fixed VoIP services

It’s fairly similar to services like Truphone or Fring (indeed, I assume Tesco’s using a white-label implementation from one of these companies or their competitors). I’ve generally been pretty bearish on the short-term commercial prospects for services, for a number of reasons, but mainly, they require far too much effort on the part of users, both for discovery and installation and regular use. Part of that’s because, of course, handset vendors don’t want to make it too easy to access these services, lest they upset their operator customers. The applications are far from seamless, but they still require more user intervention than just hitting the call button — which is too big a hurdle for many people. And that’s assuming they’ve got a compatible handset, are aware of the application, and install it successfully.

Then there’s the little sticking point that they’ve got to be in range of an accessible Wi-Fi hotspot. That’s a lot of hurdles to overcome, despite the savings on offer, which means that only the most dedicated users will bother to use it. For most people, the transaction costs are too high, or put another way, it’s too much effort for too little savings.

But there’s a decent market niche who love this sort of thing, and for which time is no object when it comes to saving money. People in the UK in that niche are likely already using a dirt-cheap, no-frills MVNO, like Tesco Mobile, or using its cut-rate fixed VoIP, or other telecom services, giving the grocery giant a built-in and receptive audience at which to aim this service. For that reason, it’s easier for me to see something like Tesco’s mobile VoIP service take off, rather than one of the standalone players.

Truphone made an interesting acquisition a while back when they picked up Sim4Travel, an MVNO aimed at frequent international travelers looking to get around onerous roaming charges. The common thinking was that they’d eventually be able to integrate Truphone with the MVNO, offering a more seamless experience that routes every call over the cheapest route without user intervention. One would have to imagine this is on Tesco’s radar, too, and again, it’s already got a good-sized base of cost-conscious consumers to show this off to — and, of course, a huge brand and marketing reach with which to do so.

But to start making good on any of this promise, Tesco’s got to expand the availability of the service to more (and cheaper) handsets. For now, it’s only available on four Nokia S60 devices, the cheapest of which it sells for about £160.