All three mobile satellite phone providers, Iridium, Inmarsat (IsatPhone Pro only), and Globalstar, provide 911 emergency calling assistance within the United States. They all work in a similar way as well. Dialing 911 routes you to a call center that then handles the call and relays/routes to local services. None of them are able to capture location information from your phone to relay to emergency services, so any dispatching will be based off of information you provide during the call. An partial exception to this would be if you were using the Iridium 9575 Extreme phone that was linked with GEOS and you had sent an SOS message, since it would send your GPS position.
In terms of emergency-only calling availability, with all of the providers, your phone has to be registered with the satellite in order to place a call and you cannot register your phone without active service. The one exception to this is with prepaid plans on the IsatPhone Pro and Iridium. If you are completely out of prepaid minutes, you can still place a 911 call, however any prepaid plan has an expiration period, so if your SIM card has lapsed beyond the expiry period, your service would effectively be inactive and you wouldn't be able to register with the network.
I hope that this helps shed light on the subject.