Instant landlines using your mobile phone

Let's face it, most people don't seem to have a landline these days. But most cable and phone services in the UK tend to try and add you a landline deal to whatever you want to sign up for. We went for it recently and still haven't got ourselves a fixed phone to actually make use of the line that now works. Which got me thinking...It would be kind of neat if there was a little dongle or attachment to allow me to use my mobile phone as our landline without us having to go buy another phone. We already have two after all. Kickstarter project anyone? Just a thought...