Definitely! Had the old BT Fusion, that integrated voip and mobile, voip calls were discounted to that of the mobile tariff, shame they ended it.
They had dedicated handsets,
As most smartphones have internet call access or apps such as sipdroid cant see why you couldnt use this with a particular logon, or as you mention even download an app (again BT are trialing a service)
Works mobile is voip linked as I'm rarely at the desk, and have ability to tie in my home phone as well all to one device #winning