call forward to extension

I am trying to set up the system so that when a call comes in it will forward to a different (outside) phone number and extension. (ir: incoming calls would be forwarded to our other location 555-1212 ext 37)

if the call is not answered it will then roll over to a different phone number and extension.

I can see how to set up an external number for call forwarding but cannot see where the extension option is.