If there is truly zero bars of coverage of any network in the area, you cannot make emergency calls.

There are two situations in which you can make emergency calls even when your phone shows no service. Firstly, if your provider doesn't have coverage in the area but another does, you can make calls using that other network. There is software built into phones that allow them to access networks that they don't have sim cards for to make emergency calls. Secondly, if you don't have a sim card, don't have a plan, or your sim card is broken, that same tech lets you call emergency services without a plan. Your sim card just gives information like your number and access to normal phone services - it's your phone's antenna that actually makes the call