If the OS doesn't recognise it as an emergency number then it may fail the call if it believes that whatever network it is associated with won't connect it ("No service" vs "Emergency calls only" indicators etc if you are out of coverage from your SIM's network but within range of other networks).
Emergency calling is supposed to work over any network (even without a SIM card inserted, much less an activated, registered, associated one), but only if the OS tries to dial it as an emergency call.
Emergency calling is supposed to work over any network (even without a SIM card inserted, much less an activated, registered, associated one), but only if the OS tries to dial it as an emergency call.