Go in with the receipt, ask politely that they fix your phone.
Listen to the BS. Do not blow up.
Ask to speak to a manager. If they refuse, demand to speak to a manager. If they still refuse, threaten to file a complaint with corporate and inform them the next time they hear from you it will be via a lawyer.
At this point, pre-threat or post-, you should be speaking to the manager. Remain calm, polite, and do not raise your voice. Inform the manager that the case you were sold broke your phone. Show them the phone and the receipt. Ask politely but firmly that they replace/fix said phone. If they refuse, repeat/say threat of complaint with corporate and lawyer. Repeat until one of two things happens; replacement phone/informed you are to leave or the police will be called. If the latter, leave, file complaint, and contact lawyer.
Hopefully you'll get satisfaction immediately and without fuss.