I have an unlocked 8700c and have installed my T-Mobile SIM. The SIM is detected and I can see my SIM info displayed correctly. However the 8700c is still pointing to the Cingular network. I have tried changing the network manually but when a network search/scan is performed, Cingular is the only network returned as the selection.
You were correct on your first response! I took the 8700 to a different location, reseated the SIM and powered on the device. The T-Mo "edge" appered by the antena icon and I can now send and receive phone calls.