It won't be in the settings on the phone, you'll need to check the carrier call diversion settings using these codes:
Turn on Call Forward: **code*(phone number to forward calls to, including area code)*10#
Turn off call Forward: ##code**10#
Check if forwarding is on: *#code#
Where: "code" is:
21 for "All calls"
61 for "No answer"
62 for "Unreachable", ie "out of coverage"
67 for "Busy"
Specifically for "no answer", you can set the duration your phone rings before it diverts using the the following:
**61*(phone number to forward calls to, including area code)**(number of seconds)#
Info from here.