In this module we will see how to return 503 if the call fails due to ANI/DNIS block
Go to Configuration → Global Route Error
Below displayed window will appear.
Screenshot: Global Route Error
On this section you can defied release codes as per you needs,
we will see for ANI/DNIS Block, just scroll the page you will find ANI & DNIS block related fields.
Here by default Block ANI & Block DNIS is set to SIP 403 Forbidden, now you can edit and set it to SIP 503 (Service Unavailable)
after filling the fields as per your need, click on SUBMIT button to SAVE.