Rather than limit use of SmartTalk to manufacturer specific smartphone apps, why not issue login details to allow generic VoIP devices / native phone VoIP applications to use the service?
Then, for example, households with VoIP capable handsets at home could make multiple calls on their landline account at the same time, and not everyone would need to have an Apple / Android smart phone. And you could still limit the number of registered devices.
It would also support the longer term strategy of removing the analogue local loop by using VoIP throughout as is the case for fibre services. People with noisy lines would enjoy better sound reproduction, and many problems caused by voice and broadband services sharing the same copper pair would no longer be noticeable.
Finally it gives BT the chance to upsell VoIP handsets via the BT shop - could be a nice business opportunity 
