I have purchased iphone 6 plus very recently. All incoming calls showing its caller ID as Unknown .. Its very annoying to use .. Please lemme know the solution for this

HI, i have purchased iphone 6 plus very revently from US & Am using it in India. Later i logged into icloude account with my old Id n password. All my contacts got synced. But the original problem is when i receive call from anybody it shows the caller ID as UNKNOWN.. I tried all possible settings to fix the problem.. Can anybody help mr with the proper solution to fix it.