AT&T SIM Card Not working in Cingular Treo

I recently purchased a lightly used Treo 600 that's branded as a Cingular device. Unfortunately, when I put in my AT&T SIM card (it's a prepaid SIM Card) the phone doesn't detect the network, even when I hold down the power / end button and search for the network.

Any suggestions?

---

› See More: AT&T SIM Card Not working in Cingular Treo

To keep this to one thread, please answer here:
http://cellphoneforums.net/att/t2955...martphone.html