Post paid mobile bill -crossed credit limit

Hi,

I am Revathy  and  is using Tata docomo postpaid mobile service. My credit limit was Rs 7500/-

I went out of country and used international roaming services and got a bill of Rs 200000/- . I  want to know is that valid if my bill is more that credit limit. I did not get any intimation from Tata docomo that I am nearing the credit limit. I tried to get this fixed witht the customer care and they told ,i have to pay the full amount . Can I file a case against them? .Is it valid to raise a bill more than credit limit. As per my understanding,service should be cut if it goes above the credit limit.

Please advise me on this

Thanks,

Revathy