Help Name not showing up...

Hi frnds i have Nokia N72 1 and half years old, evrything works fine but only one has problem. My sisters name doesn't show up while dialing or getting call😱 i tried with adding area code and country code:angry: but waste. Only this number has got the problem all other numbers work fine. When i dial this number only number appears but not the name even though the name is added in my fone bool, i have remembered this number coz i don't lift most calls, all my 300 numbers fone book works fine but this number.. Its airtel number. Once i had to pay a big price for this, i didn't lift this call thinking that this call is from is from somebody else i didn't lift it and was a big looser for that silly mistake. Plz help me why its not working