Will it be possible to make an anonymous 911 call? There can be scenarios, I think, where we (society) do not want to deter someone from reporting an emergency due to fear of being identified. Off the top of my head, consider a situation involving organized crime taking place, where call centre staff (or law enforcement) may be compelled to leak the identity of a witness.
If you want to report organized crime that involves the upper echelons of power, don't use cellphones at all.