A reverse 9-1-1 call is a call initiated by emergency services and law enforcement to telephone numbers to notify residents and businesses in a specific geographical area of an emergency and any actions they may need to take. 
This telephone notification system utilizes the 9-1-1 telephone database and therefore is able to contact listed and unlisted landline telephones. 
So if you have a landline you can expect to get the call there, but because the law was enacted before the age of cell phones, receiving a call anywhere else isn’t guaranteed, so you need to opt-in to receive emergency notifications on your cell phone. The reverse 9-1-1 system also requires self-registration of Voice-over IP (VoIP) phones before messages will be received. 
The County Department of Emergency Management has emergency-only access to reverse 9-1-1 data from local phone companies and this data is refreshed several times a year, however, it’s only as good as the data they get from phone companies, and it doesn’t include things like IP-based telephones or phones that are through other carriers. So, if you have a VoIP or cellular telephone that you would like to be notified on, you must self-register those telephone numbers. Fortunately, getting your cell number into the database isn’t difficult to do. Start here: www.smcalert.info.