Perhaps I should copy/paste earlier posts I have made, I seem to be making the same comments over and over and over again with regard to certain facts.
With regard to Kelli's phone, there are several lines of investigation and forensics that take place. The bottom line: Physical location of the phone is not as important as any technological information they have received in the phone's operation. Many people here seem to think that the discovery of the phone will make or break this case and determine where Kelli is. The reality, though, is that the physical presence of the phone is not nearly as important as the other attributes, which I will break down one by one.
1. GPS trail: The phone model that Kelli was using has the ability to trace a GPS trail independent of actual usage, and unless the phone was A) turned completely off, or B) set to Private (both unlikely), then this information would be the most circumstantially correct in determining Kelli's movements. These are the "pings" that police have released, but keep in mind that the police doesn't release all information prior to a court case unless they need help. In short, they most probably have the entire GPS movement trail of Kelli for the past few days, or at least hours before and after she was last seen.
2. Voice protocol (normal phone call) vs Data protocol (Texting, pictures, or video): Technically, every communication through a digital phone is handled in the same manner. What I call "voice protocol" is what is normally understood as a "normal phone conversation" or "voice message." But the "data packets" that are sent are done in the same manner as texting, sending pictures or sending videos. Each packet can be broken down and traced through whatever technological media are utilized: cell towers, satellite transfers, and other switching networks would be forensically investigated through whatever protocols are used. Obviously, if someone say they called Kelli at 2:30 PM on Friday, then the records on Kelli's phone would reflect the time called, the protocols used (in this case, voice), the identification of the caller's phone, and in extension, any medium used to transmit the voice message (including cell tower locations or even GPS locations of both caller and receiver). It can tell how many packets were sent, and even find the exact route of all packets.
3. Once this occurs, they can use the information obtained technologically to corroborate with witness or POI interviews. This is the true value in the technology, NOT the actual phone itself, which is only one piece of the puzzle.
Now, it MAY be important to find the phone to obtain other evidence, such as Nick's fingerprints or other POI's dna evidence. But that is a separate piece of evidence, not a dealbreaker. The bottom line, folks, is that it is not important to worry about where the phone may be and judging LE's performance (or lack of) to that end. Anyway, if you wish, get a more technologically connected poster who can describe what I surmised above in greater detail, but the above procedures would have pretty much been covered. IT forensics is turning out to be one of the most concrete evidenciary trails in LE's armory.