@foone now you have to ask yourself ... what alignment are you??
Lawful Good - Contact the carrier and return/destroy it?
Chaotic Good - Just straight out destroy it.
Lawful Neutral - Put the SIM somewhere in a drawer where it will be forgotten.
Lawful Evil - Rack up a bunch of data and THEN contact the carrier.
Chaotic Evil - Rack up a bunch of data until the SIM dies.