Hi,
you can select the WAN-Port in the Priority Order. So if you want to make it fix - select “enforced” and WAN1 for example. (I would use it with priority, because if one WAN-connection failed the devices are still online).
As I’ve written in my first post, in the Forum is an explanation for that. I searched it for you Understanding and Configuring Outbound Policy
How to specify the range … use http://www.subnet-calculator.com/ for that.
Leave the phones in the /24 network, only select it for the outbound roule - that should be enough (I never tested that - but I think it works)
Regards
Dennis