Hiss can't be coming from the outside world (noise on a digital signal doesn't sound like noise once it's been decoded), so it's something in the phone. I'd ask for a replacement. (Unless your hearing is really good. There may be some conversion noise around 44KHz, and if you can hear to 22KHz you may be hearing a subharmonic of that. But most peoples' hearing is so far down by that frequency, even as teen-agers, that it's not something really checked for in the design [because by the time you're old enough to design phones, you're too old to hear that high up even if you could when you were younger].)