False gaurantee

I bought a phone at game store. It doesn't have any physical damage,it just don't operate well which its the manufacturers fault.however I took it back to the store hence it is still under guarantee. The store kept it for 21 days, flashed all my thing's inside that phone and claimed its repaired. When I get home it stI'll does not function well. I returned to the store they said they gave it back to me because they can't fix it....whos problem is it ?..because the phone is still under guarantee and no physical damage. .they kept it so long claiming to fix it.I spent weeks without using my fone which I bought with intentions to use. I really require the store to give me a new fone .what should I do with a none working phone? What is the gaurentee for?