The Online Family answers:

Recycling your old mobile phone is a good idea -- this way it won't end up in a landfill. Web sites like Recycle My Cell Phone, Recycle for Breast Cancer and Charitable Recycling all allow you to donate old phones to protect the environment or support a cause, for instance. Before you send in your phone, however, make sure to disconnect your service and remove the SIM card (which contained your old cell number). Also, after you transfer the numbers to your new phone, then manually erase your address book for added security. Cell phone recycle programs will also give you a checklist of things to do before you turn in your old mobile phone.