Configuring a Zyxel 5501 without GUI

Hi,
I have a Zyxel 5501 router that connects me to the German Telekom network.
The Zyxel has a means to redirect phone calls and I would like to use 
this feature on and off (eg enable redirection when I'm leaving the 
house and disable when I return). My problem is that using the GUI is 
inconvenient (to put it mildly), so I'm looking for a way to do this via 
some command line utility.
Does anyone have experience with this?
Thanks in advance,
Josef