Oh very well, I stand corrected.
I don't have another headset, but thanks for the suggestion. That said, why are you so convinced that Plantronics Tech support is wrong about the features of their own product?
Tech support people are the bottom rung, least technically competent people in the chain. That's just the reality of things. We get a parade of people through here who are told one thing by Apple (or whoever) and we basically help them prove that what they were told is dead wrong. I hate calling IT people at work because I know more than they do and wish like **** I was allowed to just fix whatever problems I have myself and be done with it.
In your specific case… take the message into context. The word "factory" implies hardware, not software. Although I cannot COMPLETELY rule out it being some app you have (being wholly unaware of what you even have) I can't think of a reason why any app on the iPhone would utter a phrase like what you are hearing. They just aren't allowed deep integration into the hardware, and the message implies something to do with the headset volume. Having used an iPhone since shortly after it debuted, I've never heard the phrase come from it nor heard of anyone having heard the phrase.
So why could the headset be uttering something its not supposed to? Maybe it's malfunctioning. Maybe it's in some factory test mode that is accessed by some fixed combination of button presses that accidentally got pressed by sheer coincidence, or there is a short causing it to enter that mode. Maybe the firmware is corrupted. I had a crazy issue some time back where my old Mac Pro was hanging on boot up… it was stuck on the Apple logo. It resolved when I unplugged the USB cable for my Cinema Display. At first I thought it was the display acting up. But no… it was my Jawbone bluetooth headset. It had never occurred to me to unplug it from the display until someone suggested it because, well hey… I figured it was "just charging". Well the Mac was trying to boot off of it. Seriously. The blasted Jawbone was malfunctioning in some manner, dunno how or when it started, but the guys who helped me with that (highly competent tech heads, I should add) had seen that problem before. Now… if I had called Jawbone tech support and suggested that their headset was keeping my Mac from booting, I'd bet my life's savings they wouldn't have believed me, or insisted it was a problem on Apple's end.
But anywho… back on topic. Your problem may not be the headset, but sheer logic points to it. It's going to be a process of elimination to figure it out.