With BSNL landline, you have complete control on your phone and you can restrict any un authorized usage. For this all you have to do is Lock/ unlock either the ISD service, STD service or the Local calling facility.
First of all, you need to have a secret 4 digit code number to lock/unlock.
Choose a strong 4 digit number that is hard to guess.
Then dial 123 xxxx xxxx, where x represents your secret 4 digit code. For example, if you choose your code as 6392, you must dial 12363926392 and wait for a slightly long beep sound as a confirmation and then disconnect.
Now your secret code has been registered.
Lock STD/ISD calls: If you want to lock STD/ISD calls, you have to dial 124xxxx1 and again wait for a long beep as a tone of acceptance. From now your landline cannot make an STD call. You try calling any STD number and you’d get a response that this telephone does not have this facility.
If you want to open/unlock STD/ISD calls, you must dial 124xxxx0 and wait for the beep tone and then disconnect. You can now make STD and ISD calls.
Lock Local calls: If you want to stop/lock the local calls from being made from your landline, you have to dial 124xxxx4 and wait for beep tone and then disconnect.
Note that you must have STD facility unlocked to Lock Local calls (in other words, at any point of time, your landline must have outgoing facility; either local or STD)
To unlock local calls, you must dial 124xxxx1 or 124xxxx2 and wait for the beep tone and then disconnect.
Change secret code: In case you want to change your secret code, you must dial 123 followed by your old 4 digit code and then your new 4 digit secret code.
In this case if you want to change you code from 6392 to say 8725, you have to dial 12363928725.
If you have previously set a secret code but forgot the code, you can call BSNL customer care who will reset it to standard 1234 (which you can change as explained above).