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

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What you are probably thinking of is that you don't need credits to call emergency services. You can be at 0 cents or pence and still call them. But if there is no service, that's just what that means, NO service, your phone cannot reach a tower or call anyone

The Wireless Communications and Public Safety Act of 1999 requires any cell phone be able to call 911 on any network it can find the signal of, even if it has no service available on that network so that you can use deactivated phones or make 911 calls to local emergency services in areas that might not have official 911 service.

There is a difference between a lack of signal and a lack of service. Without signal your phone can't find a network so nothing could work. Without service your phone might be able to communicate with a network but might not be activated, or it might be another carrier's network where you don't have a roaming contract, etc. If the network is there, you can call 911 even if the phone is locked or doesn't have a contract. But if there's no network you're up a creek without a paddle.

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You don't. When you don't have service, you don't call anyone. Maybe you can call 911 on someone else's phone that has service.

EDIT: If by "service" you mean you don't have a paid subscription, then the carrier provides 911 calls for free anyway by law. If you mean you don't have service because of lacking signal, then you can't call 911. Both are a lack of service.