Micah_kurtz:
Actually, you are able to make calls without the SIM card
I think the phone service provider determines whether it will accept an emergency call from a device which does not have a subscriber identity. The phone itself is required (by the GSM standard) to support the capability, but you need the service provider to support it too in order for the call to succeed. There are many countries where service providers all support it, but some countries such as the UK where they don't. So, if you're in the UK and turn your phone on without a SIM installed, the phone will probably say something like "emergency calls only" but you won't actually be able to complete an emergency call.