I suspect the Jawbone UP24 I received is a counterfeit. In the US, there is only one size for headphone plugs and outlets. The UP24 I received had a headphone plug that was, at most, two-thirds the size of a proper headphone plug. I know this for a fact because the "UP24" I ordered was for my mother and I compared the medium "UP24" I received to the small UP24 I already owned for myself, which works perfectly. In addition, the "UP24" I received did not power on, even after hours of charging. In sum, the "UP24" I received was both damaged and defective.
I sent multiple messages to zkarlo through eBay and only once did I receive a non-form response (the form responses merely directed me to fill out a return form), and in that one response zkarlo refused to accept responsibility for the 100% defective product it sold me. I asked that shipping costs be covered for the return since I received what I believe to be a counterfeit item, but zkarlo ignored my requests by repeatedly replying with the exact same, word-for-word, message. It is clear to me that zkarlo does not care about its customers or selling proper, working electronics. Buyer beware when you shop with zkarlo.