The UK incumbent is offering straight VoIP calling (like Skype and Viber) over either WiFi or (if that's not available) over the pre-paid data connection. Users download an app for both Android and iPhone which syncs up with their onboard phone directories.

The approach chimes with BT's long-standing WiFi hotspot strategy (where its home hub users share access with other subscribers) and it comes at a time when there appears to be a swing back to prepaid mobile in many markets due to a combination of economic austerity (causing users to look for savings) and the increasing availability of capable budget smartphones which tempt users into buying their smarties outright to avoid an expensive contract, buying pre-paid minutes and data instead.

To make this 'own the phone and prepay' approach really fly on the savings front users need ad hoc options to make cheap calls and data downloads as and when they can, using their own home or other free WiFi data connections, and this 'BT SmartTalk' appears to be aimed at filling that role.

Up to five family members can link their SmartTalk app to one home phone account so in practice BT wins by encouraging households to increase the landline minute plans to cover the extra volumes.

As this is WiFi, the service will also roam and therefore offer cheap home calling services when users are away on holiday or out and about in range of hotspots.

Much will depend on how this is marketed. The problem (like all of these hybrids) is that the time taken to explain how and when the option can be used divided by the actual savings to be made (or some such formula) can make it a game hardly worth playing for many users.

In summary, SmartTalk is not about to change the face of UK telephony overnight, but it is one more penny-pinching option for users who can be bothered to fiddle about to drive down their mobile phone bills and its availability is one more possible incentive for users to switch to prepay.