I've also learned that after saying "Hello?" or whatever when answering the phone, if there's a delay from the other end, just go ahead and hang up.
I used to do that and almost got bit by it once. A guy called, and hesitated and I almost hung up. He started telling me he was from a firm that sounded like a law firm, but it was really a collection agency, and was telling me I owed money on a past due bill to the tune of about $90 ($45 for the bill, $15 for the late fee, $30 for the collection fee). I asked him for details and he just kept saying "better pay now or it will go on your credit report" and was pretty aggressive and belligerent about it. I told him to email me the information about the bill in question, as I was sure I didn't have any delinquent bills and he told me it was an Urgent Care place and he'd send me the bill. He never sent the bill, but I called the urgent care place and sure enough, I had an outstanding bill with them. It was their mistake though as they screwed up my mailing address on their end so I never got the bill and subsequent notices. They said they'd fix it, send me a new bill for $45 and to ignore any late fees and they'd handle the collection agency. I paid the $45 while on the phone (as it was a legit bill) and hung up.
Two days later, the collection agency guy calls and says I owe him $75 ($45 for the bill and $30 for the collection). I told him I talked to the urgent care and already settled it. He said it doesn't work that way, they can waive the late fee, but that's it. He got very belligerent with me, screaming at me on the phone, threatening to ruin my credit, etc. if I didn't pay up, and was being a total ass about it so I hung up. Never heard from him again, nor did anything show up on my credit.
I get it, those collection agencies must hear "I never got the bill" or "I already paid it" 90% of the time, but man was he over the top. Now, I at least give them a second longer after the initial delay before hanging up just to make sure.