The battery in your cellular phone contains many toxic agents that are very harmful to the environment. By recycling your old cell phone, you are preventing these agents from contaminating the soil.
If you want to donate old cell phones, the following organizations are dedicated to refurbishing them. They make a donation to charities for every cell phone you send them.
Also, many cell phone carriers will actually give you a rebate for returning your old cell phone. Get in touch with them, you might be surprised.