Do you have any voltage on SIM_VDD with a 1.8V SIM card? The module applies 1.8V to the card and tries to communicate with it. If that fails, it then applies 2.8V and tries to communicate.
Have you tried SIM cards from different vendors? I've heard of problems with Telit radios not working with cards from certain vendors. I've never heard the problem reported with SIMCom modules though.