If you had a one year contract your contract would be fulfilled in October 2005.
When Cingular assumed AT&T Wireless' assets they also assumed all of its agreements which would include any contract you had with AT&T Wireless. If you had a year's contract with AT&T Wireless your contract did not end when Cingular purchased AT&T Wireless. I'm pretty sure that if you had failed to meet the terms of your contract with AT&T Wireless they would have charged you an early termination fee. Bottom line is that you are responsible for your contract til the end of your contract term. There is no get out of jail free pass by your being absorbed by Cingular.