Cell phones contain such toxic materials as cadmium, lead, and mercury. Millions are already in landfills, contaminating groundwater, and causing numerous health problems, such as birth defects.
If AT&T offered people a $5 rebate on their phone bills for turning in used cell phones, this would help the environment -- and AT&T could then advertise this fact to make themselves look more eco-friendly.
If these phones can be used or repaired, AT&T could do exactly that. If they are beyond repair, AT&T could still melt down the materials, and re-use them to make new phones. The good public relations from doing this would make such an effort pay for itself.