How do cell phone cards work? Do you have to join a plan to use one? What about compatability with Cingular?

AT&T has discontinued wireless service. We have a Nokia phone that is Cingular compatible. Can I buy a $15 phone card at Walmart and expect it to work with my old number instead of joining another plan?
Categories: Phone Cards Tags: about, cards, cell, Cingular, compatability, join, phone, plan, work