At a previous company, we hired United Mercantile and they never collected a cent from either of the two accounts we sent them.
They made things up like saying the one guy's corporation was surrendered when it wasn't and on the other one, I later reconciled with customer and he said he NEVER received a single call or letter from them.
Both were relatively small amounts (about $5k and $10k) and it would appear that they never lifted a finger to get their 30% or whatever it was.
I like the lawyer approach -- the more names on their letterhead, the better! :) No company likes the threat of legal action.