I have been reading up on the subject of forensic cell phone records analysis. From what I can tell, if Jennifer or the brothers friend's phones were not Smart Phones, there would be no "historical" record of where the phones were when they ping or when they were when they were powered up or powered off. What would be available would have been the cell tower the phone was "engaged with" at the time there was data transfer with that phone; i.e. an incoming or outgoing call or text message.
The cell tower the phone was "engaged with" would be the tower the phone got the strongest signal from. Very often this would be the nearest tower but sometimes companies will mix towers with different signal power to achieve maximum coverage. Either way, the phone company would have a pretty good idea of the geographic area a phone was located in by what tower the phone was engaged with during normal operating conditions. During periods of very heavy usage or if there were problems with a tower, including routine maintenance, conditions would not be normal and a phone might default to a tower that is near but not in the predicted geographic area. If a phone is in a moving vehicle while in used, the tower it is engaged with might change and a record of that change would be retained. If a phone is powered off, loses its signal with all towers or is simply "on" but not being used, there would be no way to tell from the historical record of where the phone was at any given time. If a call or text message it made to a phone that is "engaging" with a tower, a record of the tower would be created. If a call or text is made to a phone that is powered off or out of range of any tower, a record of that failed call will also be created.
Apparently the FBI has determined that now, 90% of calls go to the "predicted" cell tower. It is possible that 12 years ago, that number would have been different. Unless the cell records are evaluated by a technician who had access to tower volume and tower efficiency, an estimate of a phone location might not be particularly accurate.
The significance to Jennifer's case would seem to be that LE would be able tell the general location of the phone when she made her final calls Monday night but, to be accurate, the review of the records would have to include consideration of signal strength from the predicted tower. If a call engaged from a different tower, it would be difficult to differentiate between a call that defaulted to a different tower and a phone that was actually in a different location.
Law Enforcement would not be able to tell when a phone was shut off but, when the first incoming call came in, they would know that the phone was powered off. (It is my understanding that cell coverage is pretty good in that part of Florida so someone would have to drive quite a ways to be out of range if any tower). We know that her boyfriend called her Tuesday morning and the phone was "off". It would be interesting to know if there were any incoming calls or texts before that. If there was an incoming call at 10:40, and the phone was off, it would be highly significant. Otherwise, it do see where that time would have come from.
I suspect that the phone records may not be that important. The first incoming call or text after the phone was powered off gives us some idea of the timeframe of whatever happened. For someone to take the effort to power off a phone during an abduction would suggest a high degree of criminal sophistication. If, for some reason Jen powered it down herself, possibly because she was mad at her boyfriend, it would have created a classic "red herring".
Incidentally, if any of this technical information is wrong, please let us know.