Your ONLY option is to call wherever you ordered it from and hope your sweet-talking skills are up to the task. Frankly, I'd be shocked if they got you the rebate - the original UPC is required in order to prevent people from submitting duplicate requests and there is no way for you to prove that you're not doing that.
The IMEI number is on the UPC, so there's a faint possibility that they can look it up like that to prove nobody has submitted a rebate on that phone, but that's only if they actually track the rebates by that number.
Call and be really, really nice and insistent.