Your coworker and your sister have a good shot at hosing this collector in court, assuming it's really Shum's office calling and not some overseas PDL thug ''borrowing'' the firm's name. You may have clouded the issue by posing as the alleged debtor. Technically they discussed the debt with the wrong person, but I'm not sure you have that violation since you gave a false identity.

The crap about being arrested, that's something no one should hear from a debt collector. If they let five days pass from that first contact without sending your sister a dunning letter, rack another violation toward a $1K statutory award.

Learn how to exercise your FDCPA rights as an alleged debtor at FTC-dot-gov.
 Feb 14th, 2012
A lady named Megan called and stated she was from the Law Office of Richard Shum asking to speak with my sister about some legal documents. I was familiar with this company because my co-worker had someone from the same company calling our office 3 months ago. They were threatening to come to our job and place my co-worker under arrest with the police. My co-worker got so frustrated that she told them the when she would be working and gave them to schedule to lock her up. Needless to say, after she got nasty with them they stopped calling her. Back to my sister, I pretended I was her and they started to say that I owed them money for writing bad checks. My sister doesn't even own a checkbook so I knew this was false. I played along with the lady and asked her to fax me documentation at work and I would pay the $687 immediately. She said the fax machine was down and if I didn't pay it at this very moment then I would be served by the police. I said I wasn't paying anything until I had official documentation. It's been almost 2 weeks and I haven't received another call, received anything in the mail, or a fax. This company is a complete scam and DO NOT GIVE THEM ANY MONEY!!!!!
 Feb 12th, 2012