How can I achieve operator agnostic call barring?
That's probably not possible at the moment, as you'd have to forward the calls using VoIP. This is way too complicated to implement on your own, as these features are already builtin to the mobile network.

Alternatively your MNO may have some USSD codes for automatic call forwarding, so you need to look if those are available to you. 

Unconditional Call Forwarding is usually activated with:
*21*[phone number]# and cancelled with ##21#. For other options, check the link.

Call Barring:


  If you wish to restrict calls that can be made or received by your handset then this is referred to as "Call Barring". All Call Barring codes are specific to your network, so you'll have to ask your service provider


Also check this