I'll bet, though, that there might be some non-traditional ways to try to find such a person. For instance, if the PIN for the phone was paid for using a credit or debit card, that might be traceable. If the phone is an iPhone, iCloud might know where it's been and perhaps Apple could be talked into cooperating. I have an iPhone on a prepaid network and so do family members, and I can "Find" them through Find My Phone capabilities.
Also, if they could identify geographically what towers were pinged by that phone, and watch the pings over a few days' time, there might be an obvious pattern, such as the phone is always in the area of some fast food restaurants from 6 a.m. - 2 p.m., which would clue you in to the fact that he probably worked in that area during those hours. Narrow down who worked those shifts in that area, and then see which one lives in the residential area where pings happen at night.
There are also records somewhere showing what numbers that phone has interacted with. If they can find several contact phone users that have engaged that number...
I'll bet that surely it can be figured out.