I dont see the update to your answer. Just to recap this is the only extension with the issue. I have compared it to all the other working extensions and there is nothing I can see that is different.
What he's telling you is that you need to set the "Call-forwarding-on-no-answer" feature. The ACFS is the command to do that.
If that command, or how to apply it, is unfamiliar to you, it would be worth your time to have a certified NEC tech come out and set it up.