(one of those things I've seen very little discussion of, the WP page correctly points out that this mandates a mobile-station in every car; although it does not precisely mandate that it be always-on, in practice it will be in order to manage messaging promptly)
This is no longer reliably the case, because the carrier is legally required to provide a minimum set of information about the caller (its location) to the emergency services, which many cannot fulfill if the call was made without a SIM.
In the past they handled the call without that information, but after an incident in 2013 the court ruled that the requirements also must be fulfilled without a SIM (0).
So some carriers (notably all German ones) stopped accepting Emergency calls without SIM, first to not be in violation of the law but nowadays apparently due to "misuse" (?) (1).
(one of those things I've seen very little discussion of, the WP page correctly points out that this mandates a mobile-station in every car; although it does not precisely mandate that it be always-on, in practice it will be in order to manage messaging promptly)