A couple thoughts...a few bad ideas and methods but you can be the judge of what's necessary in your situation,
One...Google their name and address...it's sometimes surprising what sites someone's listed their number on! .
Two...if the person's had the same number for a while but only recently unlisted it, it may be in an older directory? .
Three...find a close relative and call them for it w/ the old chum story...better yet, have a girl call as Mom or whoever doesn't get the threatened feeling before they let it out? .
Forth...more deceitful but if you know their work number call them there (again a girl works best) and pretend it's the bank calling w/ the usual qualifying questions...ask their mother's maiden name, confirm their work number then their home number...when you've got it, hang up while you're speaking as everyone thinks you've been cut off as who'd ever hang up on themselves in mid sentence?? .
Lastly and even more deceitful and illegal...if they have a house mailbox, drop off a 'sales flyer' at the house before and after theirs and cruise their mailbox for their phone bill on the day it should be there...they can get another if it goes missing. .
To confirm, these ideas are personal thoughts and in no way associated with this site, it's moderators or members...please remove this posting if offensive. .
~ Pete
PS - I learned the hanging up while speaking trick from an old GM Canada Head Office Customer Service guy who told me he used this trick to diffuse conversations gone bad with irate 1980 X-Car customers...he'd call back right away and laugh about how they'd been cut off and usually the opportunity to resolve the situation resulted...and it does work! .