Hopefully whoever installed it left you a guide of how to change it. Failing that you will need a serial cable to see how the main menu is programmed and what prompt it uses.

Forwarding can be done depending on how the system has been programmed but that model only has 2 modes so depending how day and night is programmed you may need to do quite a bit of reprogramming.

I'd get a tech in and get him to give you the prompt numbers and tweaking the system to allow forwarding.