The scheme is being run by electrical shop Comet who will give people a free pre-paid envelope to pop their old handsets in.
They will then be sent to a recycling firm where they can be taken apart carefully. Plastics and precious metals that make up handsets will be reused for other things.
Around 45 million people own mobiles in the UK.
But it's dangerous to throw old ones away because some bits in them could harm the environment.