The ToneFone is basically a chunk of steel wrapped in a rubber shell, and if not for the fact it's wrapped around an iPhone it would simply be another exercise weight.
Sounds simply, right? Here's how it's described on the product page:
The ToneFone is a revolutionary new case for iPhone 5 and 5s that turns your phone in to a weight loss device by increasing its weight to up to 1 or 1.5kg - allowing you to work out wherever you are.
It’s simple: you get a text you get fitter, your mum calls you, you get fitter, you check Facebook and you get fitter, even when you order a takeaway you get fitter. You can take it with you anywhere and every time you pick your phone up you’ll be working out.
This case has proven more popular than the retailer expected, and it's sold out at the moment. They're taking pre-orders for the next shipment, in case you're interested. It's only available in white, though gold and grey models are due to be released in June.