warranty is thru your phone provider primarily but headphones get a 1 year warranty.
Monster themselves, after the space of the first year are kind enough that they will replace the headphones once, no questions asked, but a receipt is needed ( i dont know whether the phone contract will work in this case, as you have been "gifted" the headphones by HTC rather than purchase them yourself)
---------- Post added at 02:46 PM ---------- Previous post was at 02:40 PM ----------
oh and also just for a heads up, monster are a great sounding headphone, but even the £349 Pro headphones they make in the beats range, are prone to being faulty.
I work at a retail outlet and over the last christmas, Beats by Dre were probably returned 1 in 10 purchases as faulty. Most of the time this was a problem caused by the wire itself rather than the headphone driver.