Don’t just leave it there at the back of your drawer, and don’t just throw it away! If it is in working condition, consider donating it to a charity or selling it if you want to make some money.
If it is too old or broken, recycle it! There are specialised recycling facilities for electronic devices and many mobile phone shops will recycle it for you. You would be suprised to see how many parts of a phone can be recovered and reused 😉 Easy!