Hi,
You will have to navigate to Services > SMS Utilities and go to Call Utilities tab, there you will find the checkbox:
Some of the users have reported that they have issues with calls not being rejected with the latest RUT240 firmware 1.12.3. If you're going to have issues too and it's really necessary to get it working - please create an account in the forum and private message me, I will send you a TEST firmware for testing purposes.
EB.