I cannot say, with 100% certainty, whether most callers are telling the truth, when they tell what they do or have done.
There are some things that sound more extreme or unlikely than others—anything involving horses or donkeys, for example, is probably right out—but unless their stories involve shape-shifting, tentacles, space travel, or otherwise defying the laws of physics, I can't definitively rule out their veracity.
Now, I am certain that there are enough actual occurrences of quasi-consensual incest and horny German Shepherds and sure, why not, unprincipled babysitters and furtive adult-video-palace back rooms and BBC-themed parties complete with silver-spade anklet charms, that almost any of these scenarios are within the realm of possible. But since it doesn't matter whether the scenario is real, except in the case of underage kids and animals, I don't generally expend too much energy worrying about it. There may be tiny to medium-sized holes in the narrative, or little notes of costuming or language that are jarring, but my mind automatically glosses over these inconsistencies. I assume it's real, for the sake of entering into the spirit of the thing.
Occasionally, though, something pokes through a caller's supposedly true story, a detail so hilariously glaring and out of place that I'm just, like, oh, for fucks' sake, I can almost identify the porn site where you watched this. Especially when they take great pains to assure that everything they're about to say is Absolutely True and Real-Life.
My most recent encounter with this would be the fellow who wanted to talk about how he and his wife had been swingers for ages, and had started visiting the "BBC room" at some of the parties. Fine. That's fine. I don't doubt it. I'm sure such places exist. He went on to say that his wife had gotten a black lover, except he said "she has a BBC". If I took that literally, that would have jumped the rails over onto the black "she-male" track; in the spirit with which he offered it—his wife has a boyfriend with a BBC—it's an incredibly objectifying construction, but I wouldn't put it past a white submissive entitled douchebag to think of his wife's African-American lover in that way.
"He's a real bull," the caller went on, showing how down with the jargon he is. "He's bringing his whole posse, I guess they call it, over tonight." And then he dropped the ball so thoroughly I couldn't recover. For the entire rest of the call I nearly had to laugh. "I love being a cuckold," he said, except he pronounced it "coo-cold". He had clearly never spoken the word out loud in an environment where anyone might correct him. He had clearly only ever read it.
"I love being a coo-cold."
I don't even know how to end this post. I'll just end it there.