Callouts, Pager Trees and Phone Trees.
Each of these and several more types of Call-Outs for an Emergency which have not been included here are by themselves not necessarily the correct answer for how to notify anyone of an Emergency. There are inherent problems with each of these types of Call-Outs which do not take into account various 'Murphyisms' or 'Murphy's Laws' that can and do happen.
Regular testing of several types of methods for announcing an Emergency should be done periodically in the course of a year and without letting a vast majority of the people know when they might happen. Sure you can give them a rough time frame say a weekend beginning Friday evening at 6 PM local and ending Sunday at 6 PM.
Obviously, while Emergencies can and do happen at anytime, you want to make sure that a test happens during a time when you can be sure to get the most participation. Three O'clock in the morning when almost everyone is asleep is a sure-fire way to set-up the exercise to fail.
Phone Trees rely on the simple fact that the Telephone system is still a viable communications device. Pagers can also rely on the telephone system or an Internet Connection which can or cannot be a telephone line also being available. Paging like some Fire Departments do via a local Repeater are an option if you can get a Repeater Controller to generate these tones.
Getting on the local repeaters and/or the prearranged and agreed upon frequencies relies on the simple fact that a majority of people regularly monitor those repeaters or frequencies. Everyone has a life beyond ARES/RACES/ARPSC. Odds are that on a week day, between say 8 AM and 5 PM your participation will also be somewhat limited to people who work afternoons or midnights, or retirees.
This all should be an on-going process and documented in the plan for your individual area such that all who read the plan and are members of the local ARES/RACES/ARPSC Group are aware of the various ways in which a Call-out might happen.
Test your systems today if you have not done so in a while. You may be surprised one way or the other with what you find as the end results. In the long run you and the people you serve will be all the better for the additional training that these Call-outs do.