Actually, the phone doesn't even need a sim card to make an emergency call. At least not in Canada, but I'm sure the US should be the same. If the phone is not

Actually, the phone doesn't even need a sim card to make an emergency call. At least not in Canada, but I'm sure the US should be the same. If the phone is not connected to the network currently, it will do network hunting, and will be able to do an emergency attach to the first network it finds. This may also happen if your say out of coverage with your own provider, but can see signal from another provider, even if there isn't a roaming agreement. 911 get's interpreted by the phone in a special way, and the phone will flag the request as for emergency services when connecting to the network, so the network will treat it with priority. In theory, even if you can't be delivered a phone call, or the network is overloaded, emergency calls should still work, depending on exactly what the problem is of course. It's also something that at least here in Canada, get's tested fairly extensively, both on all the devices we sell, and the network equipment when deployed and each patch. * my views are my own, and do not reflect those of my employer.