If you are absolutely certain that you didn't accidentally make the call, then next time someone from the 911 system calls you about a false call, ask them to assist you in putting an end to this. The 911 people should know the approximate physical location of whoever placed the false 911 call based on cell tower location. If you weren't near that location then you've just proved that you couldn't have made the call. You and/or the 911 system people may need help from SO customer service to track down how these false calls are occurring.

False 911 calls are a serious issue. Not only do they waste first-responder time that could prevent them from servicing a legitimate 911 call but, depending on how these calls are happening, may prevent you from getting help when you place an intentional 911 call.