Cingular, now AT&T prepaid phone from SanFrancisco will now work in NY?

Hi, so I bought a prepaid Cingular cell phone last year on San Francisco. Dont use it now as I live in Colombia. Now I plan to travel to NYC. Will this phone work there? Do AT&T (formerly Cingular) cell phones in the US cover the whole country? just add money to the phone through AT&T in NY?