Sorry to hear your new phone got stolen but, saying it's unfair that you still have to pay for it is a bit too much. Since it got stolen from a secured place you might want to take it up with that place. If you have find my phone enable you might try to see if you can locate it. If not all you can do at this point is report it stole and have it blacklisted.
You can break the contract and get a new phone but you will have to pay off the device balance, if you are unable to pay it all at once you can reach out to the Credit Operations Team and arrange a payment schedule.
Thank you for responding. Yes I did backlist it and report to the police. I talked with the customer service and got to know there is nothing they can do about it as the gaurantee is dealt by a third party. All I can hope right now is that the police find it.