Even if they have a net kill rate, it's a tragedy of the commons situation.
Regarding the safety thing: Each cell phone makes it safer for the individual. Each other person's cell phone makes it less safe for you (net: they could use it to call when you have an emergency, it's more likely they'll use it to text while driving and get you into an accident).
If you stop carrying the cell phone you're less safe (and we can debate how much less safe, it doesn't really make you that much safer, and I agree that safety is an illusion). It may be better for everyone to stop carrying them in terms of overall safety, but since that won't happen it's better for you to carry one than not.
Further, even if they do have a net kill rate: so what?
Their primary use isn't safety. A car baby seat that killed more than it saved would be an issue, because that's its main goal.
A cell phone's main goal isn't safety, it's one tiny subset. Cars probably have a net kill rate, more dying in auto accidents than are saved by rushing to the hospital (versus the amount of time it would take to rush them in a horse and buggy). But their primary use isn't to make us safer. If you want to be safer, you wouldn't step into a car. The primary use is transportation, and we accept the trade off of some deaths (and try to reduce them where we can).
A cell phone's primary use is (insert whatever you believe here - but I doubt you believe it is safety) and so we trade off some net overall safety for that use.
We don't have to, and we should try to reduce those risks (texting while driving and such), just like we try to reduce car risks (via safety mechanisms like seat belts, things like speed limits, making drunk driving illegal, etc.), but you say cell phones may have a net kill rate, I say: so what?