said by WhyADuck:
However, what your post fails to address is what happens when a cell phone caller dials 911.
Cellular services are now handled just like land line phones in part to enhanced 911 but also due to features that are part of the newer digital networks such as CDMA, TDMA & GSM.
The callers 10 digit number is now transmitted constantly in addition to the ESN which provides user authentication as well as ANI info.
Older analog handsets were not designed to transmit ANI information which is why call trace services where generally ineffective at tracking someone. For many of you who can recall back to cellular calls from more than 5 yrs ago ... they rarely if ever would show up on a Caller ID system.
Many of the wireless areas not covered by E-911 are still not serviced by digital services or have older equipment which are not compliant.
Problems with VoIP is mostly in dealing with the service providers network and if they are properly tracking and transmitting the end user's ANI to the PSTN (usually for cost reasons).
For those of you who are concerned with reaching the proper number for emergency services, I highly recommend following WhyADuck's advise and locate your local Central Dispatch number or the direct emergency numbers for Fire, Police and Medic especially if you either do not have cellular service or can not get a signal from your location.
And as JJV has noted (I experienced the same here in Seattle), that many direct or local numbers will now transfer you to 911 to consolidate, track and record all in bound calls.