I have heard of a lot of people complaining to me about not being able to use opera mini on their Nokia handset with MTNL, on calling up MTNL they just say that the problem is out of their scope and as long as the phone browser works, they cannot do anything to help you and say that it is an application problem. They are however as usual just plain liars saying they don't know what you are saying. So here's the solution:
When they send you the settings they do not send you separate settings for the access point as they incorporate it within the GPRS Setting, You have to do the following:

1. Select Settings from main menu.
2. Select Configuration settings.
3. Select Personal configuration settings.
4. Press Add new.
5. Select Access Point
6. In the address specified type:
For Postpaid enter gprsmtnlmum
For Prepaid Enter gprsppsmum
7. User name and password are mtnl and mtnl123 respectively
8. Now go back and activate this setting.
9. Go back once again and select activate default in all applications.
10. Now it will definitely work