I never was a supervisor, but I did some telemarketing as a teenager. Supervisors are responsible for recruiting new hires, motivation (it's a very grueling job with a high turnover rate), and lead generation. They are also responsible for assigning "leads" to the reps. "Leads" are either pages of the phone book(I frequently was given copies of the actual white pages) who are "cold calls" (people who've never donated) or people who have donated in the past and they are called again yearly for repeat donations, or for another client to donate to another cause. These people who have donated before are called "reloads". Also, there are "nonpubs" or non-published numbers. I couldn't tell you exactly HOW they got them(they may have contracted a company to provide the info) but my best guess is that they used a computer to take all the published numbers and fill in the blanks, because the numbers I was provided with wree sequential. Telemarketers generally make minimum wage + commission.
Supervisors also "verify" pledges for money that the rep makes, and are responsible for "the close" or closing the call. During my stint, most of my supervisors were really good reps that were promoted. That just meant they could talk really fast had an answer to every negative response. They are good salesmen. Same vein. Telemarketers are given a "script" to read off, and they are supposed to stick to it like the bible at first, until they are comfortable enough to give it thier own style, but are still encouraged to "stick to the script". Supervisors also monitor calls and coach the telemarketer. By law, telemarketers are supposed only call between 9am and 9pm. As a general rule all calls are recordered. supervisors report to a general manager/lead supervisor, who is usually the owner.
That's the extend of my knowledge. I'm glad all that time I wasted doing it finally has a use.
EDIT: here's a good story to use for your book, but I want credit in the book

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Supervisors are supposed to be "upbeat" and keep the mood light, as it is a very painful job mentally. You deal with a lot of rude people, as I'm sure you can imagine. Not to mention tons of rejection.
One of my supervisors was a real character, and as the supervisors would handle the hiring and firing of the reps, one of them had a unique way of handling it. Generally a rep got let go if he wasn't making at least 3 sales a day. that was the hard and fast rule. Everyone had good and bad days, but if you had one really bad week, you were gone. That's the game of sales. Well this was a bunch of young teenagers basically, and some 20-ish guys. So he'd come into the room and sit down next to the guy. He'd pull out a quarter, look him in the eye and say "I'm going to flip you for your job. You call it- if you win, you come in tomorrow, if you don't, you're packing it up". The first time I saw it I thought he was joking. But he did it a couple of times. He would also use that if someone was screwing around-which happened quite a bit in a room of teenagers. So he'd frequently "flip for the job". And he was true to his word. Needless to say if he walked into a room tossing a coin, we all knew what was coming.
You can't make up stuff like that- life is stranger than fiction.