Consumer Redressal

I bought a new Mobile phone Samsung S6 Edge from The Mobile Store, Vivek Vihar, Howrah with pre booking but on the very next Day I got a problem of heating during making a call, so I returned the mobile to Mobile store and they found the same problem they told me that the will replace the phone, but afte 12 days spent they are not showing any initiation rather telling me and harassing me that I will get the phone again after 10 days that is also not sure as they are telling me this for last one week. And my credit card bill is going to generate and my 1st EMI will be deducted but till now I didn't got the opportunity to use the phone for single day and the officials of Mobile store telling me that I have to bear the whole amount of 1st EMI. So what I have to do now?