I have also faced a similar issue.
The problem is valid but there is a workaround.
Assume you have BSNL cellone connection in Chhattisgarh state.
Once you login to BSNL portal, go to "CELLONE BILL PAYMENTS" and then "VIEW / PAY OUTSTANDING BILLS".
In select circle screen, select state as "AndhraPradesh".
It would show you "No outstanding bills" message if you do not have any registered connection from AndhraPradesh state.
Then just select your relevant state. Ex: Chattisgarh. It will ask for confirmation to change state. Select yes.
It would display outstanding bill if you have registered connection of Chattisgarh state. Click on invoice number. It would display outstanding bill.
Click "Make Pay". It would redirect you to proper screen with list of banks to choose from.
I am able to make payment for Cellone Chhattisgarh by following above steps. Also, I am getting 1% discount on next bill for a online payment.