Not returning the cellphone given for servising in warranty period

Location/place: Varanasi

Name of company/service: Karbonn mobiles

I purchased an android cellphone i.e. Karbonn A2+ for my sister from a nearby retailer.After using the mobile for just one month it suddenly stopped functiong.The next day I went to the service centre ,the guy noted my complain and assured me that i will get back my mobile in one month,but today when i called him to ask ,that can i come and collect my mobile ?he answerd that ,DEAR SIR U WILL GET YOUR CELLPHONE AFTER 15 DAYS . Pls tell me what should i do to get my cellphone?