Several charities accept old phones for donation, though it's worth remembering that these groups likely won't physically give your old phones to people in need. Rather, they work with phone recyclers and sell your donated phones to them.
A nonprofit group called Cell Phones for Soldiers will take your "gently used" phone and sell it to recycling company ReCellular. It will then use the proceeds to buy calling cards for soldiers.
The National Coalition Against Domestic Violence works with another recycling group in a similar manner. About 60 percent of the phones it collects are refurbished and resold. The money goes toward supporting the coalition. The remaining 40 percent of the phones are recycled, according to the group's website. It pays for shipping if you are mailing three or more phones.
Founders of Cellphones for Soldiers shown at left.
Credit: cellphonesforsoldiers.com
Related: A few more suggestions