In countries where the Emergency Services can be called without a SIM card, what caller ID is shown to them when called from an iPhone? In countries where it's possible to make calls to emergency numbers (911/999, etc) from an iPhone that has no SIM inserted, how is it possible for the emergency services to track a device - would it be possible for them to call you back, even if the phone didn't show a caller ID due to one not being present?
No - when you don’t have service, they can’t call you back.
Yes - you can call out and the only location data will be which cell tower data is provided to EMS / Emergency Services - not the GPS or location of the device doing the calling.


In more details:

A SIM doesn't determine if you can connect to a cell tower, the phone's antenna determines it.  The SIM only has your phone number and carrier info so you can make emergency calls without it.  In the case you've mentioned, the call will go thru but the phone number and location will not be sent. 1  

So in this context, whether you have a SIM inserted or not is irrelevant.  Calls to emergency centers are treated the same with or without a SIM.

Emergency calls are routed to a Public Safety Access Point (PSAP) for all carriers. Location data is sent with accuracy between 50 and 150 meters depending on if it's using cell tower triangulation or mobile-based location (GPS chip). 2