Calling Card: This small cellphone can hide behind a credit card. Photo by Modu

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Carry a different phone for every situation, whether you’re traveling light or blasting tunes in your car. On its own, Modu is one of the smallest, lightest cellphones yet. But when you want to do more than basic calling, pop it into a new outer shell to give it features like a full keyboard, a wider display or a longer-lasting battery.

Modu fits into a slot in a shell and controls the shell’s components as if they were its own. The shells will sell for $30 to $90, and the company will let partners design custom versions, such as a model from Universal Music that could have dedicated buttons for playing songs.

Modu can also add a wireless connection to other gadgets. A camera with the appropriate slot could use the phone’s celllar link to e-mail pictures, for instance. Modu launches in Europe in October with several shells that are yet to be announced. It is set to hit the U.S. late next year, when you should be able to plug it into basic shells as well as devices, including a Magellan GPS unit and a Blaupunkt car audio system

Click here to see some of the potential forms the phone could take.