It is possible that Jessica was talking to someone on her cell at 7:31 from that scene, in which case LE can definitely state that she was on the scene at 7:31. I believe LE is a lot closer to solving this than we want to give them credit for. After mulling it over for a couple days now, I am now convinced that LE does in fact know her whereabouts between 6:30 and 7:30. There is just no way that they are clueless, with everything else they have gleaned with very advanced cell phone tracking technology. There is a very real reason why they are publicly stating that they do not know where she was during that hour, and I think it is because they want to catch who they already know killed her in a lie that they cant alibi their way out of. JMO
I hope you're right in the last part of your post, but I respectively disagree with the first sentence. There is no way, IMO, to tell WHO was talking on Jessica's phone at that precise time--only that her cell phone was in use by SOMEBODY, unless the conversation was being recorded and it has been verified to be Jessica's voice on the recording. It could very well have been Jessica talking on her phone, but it could very well have been someone else as well. I'm not trying to be argumentative, but this precise time and her precise location bothers me and sets off my hinky meter big time.
Now, a question...but please don't take it as a rumor because I have no knowledge nor have I heard any rumors whatsoever that this could be the way they knew where she was at all times during that day except that one missing hour, but does anyone know if an ankle bracelet gives detailed times and locations of the person wearing it? Just a wild thought.