Can't use it on Verizon because of incompatible technology. Cingular (now At&t) uses GSM and Verizon uses CDMA. Kind of like trying to tune in AM on an FM only radio, similar technologies but not the same.
I have an Audiovox 5600 smartphone that I got through Cingular 2+ years ago and I am now out of contract. It came with a sim card. Does that mean I can use it under any network? eg. Verizon? I find that Verizon does not offer as many selections for decent smartphones.