Cell phones, land lines and physical self defense classes do not make people safe. Sure, all of these things have been used by some people in ways that may have made them survive some kind of an emergency, but they all have a tiny, tiny contribution towards being safe. If someone really wants to kill you, calling 911 or texting a friend will eventually tell the authorities about it, but it is extremely unlikely to help you survive. Fortunately, the cases where people are stopping at nothing to kill people (or otherwise damage them) are extremely rare. In order for your children to be safe, they need to learn how to keep from getting victimized by regular people: learning how to survive a Columbine style shooting is extremely unlikely to do them any good whatsoever. What keeps people safe in the real world is 1) situational awareness and 2) the ability to de-escalate situations (or, better yet, prevent them from escalating in the first place).
Very young children do not have situational awareness are are not good at de-escalating situations. So, in order to be safe, they need to a be with a responsible person who can do both of these things, and the child needs to be able to follow directions when necessary. If anyone (including a little child) needs help right now, what they need to do is yell for help. Good lungs and a willingness to make a scene are much better at getting timely help than a cell phone.
Older children should learn not to escalate situations, should retain the ability to yell for help when needed, should learn how to run fast (to get away), and should learn situational awareness to keep safe. A cell-phone has a minuscule effect on a teenager's safety, and may make them more brave, more willing to go unsafe places and do unsafe things, because, in the back of their mind, they know they can always call for help with the cell phone. Unfortunately, the help you call with a cell phone can take a long time to get to you and is therefore much better at cleaning up messes than preventing them in the first place.
Again, I think it makes sense to give kids cell phones in many cases, but I think that the mental association between cell phones and safety is quite dangerous.
To those of you who teach in classrooms without phone lines: how are you expected to talk to parents on the phone? Also, has anyone taught in a classroom in the last 20 years without at least the ability to page the main office? Doesn't everyone who teaches have an "in case of emergency" plan that includes a mechanism to summon help?