You've got to either be clever with social engineering, or have the money to pay for someone who can do it for you. That $100 rate is for people who don't even care to search the internet for good sources
You could easily find a place who will do it for a lot less, like $15 - $25.
For cell phones, there's not going to be any sort of legitimate free internet lookup or anything. Landlines are much simpler. Also what complicates things for mobile numbers is Wireless Local Number Portability (WLNP) which means you can normally figure which switch a number was originally homed off of, but not necesarily anything else because they ported their number. However if they did port their number, then it will generally just give you an idea of the metro area where the line was first bought. If you give me the area code and exchange (first 6 digits), I can tell you the original account calling area. But if they had their number ported, who knows...
BTW why do you want this info?? I get nuisance calls all the time, why don't you just ignore them!! Or program that number out of your phone, if that's what's going on.