Nokia 3105 not working after conversion from postpaid to prepaid

Hi all, i'm a novice as far as mobiles are concerned. I bought a Nokia 3105 a few years back while i was in mumbai on Tata Indicom postpaid connection. Some years later i moved to hyd and converted the piece to a prepaid connection. However, this prepaid connection worked without a sim. I suppose they changed the software. After a couple of years when i shifted back to mum and bought a prepaid card here and tried to use this cell, it's not accepting the new sim. I mean even when i put in the mum sim, i'm still on the hyd network. I went to Tata Indicom service center and they're demanding Rs. 500 to change the software. I don't think it's worth spending that much on this old cell. However, i still wanna use it as it's working fine. Can anyone help me on this?

Thanks in advance.

hi friends i am using nokia 3105 cdma mobile but i forgot its security code how to unlock that phone pls help me