Just got off the phone with Xnet, spoke to both sales and tech support.
I understand that a toll bar can be set up at a $5 cost. I'm also told there is no workaround to allow for flexible/selective toll barring i.e. turning on and off, PIN restricted etc.
This thread back in 2007 looked promising:
Perhaps back then there wasn't even a toll bar function.
Using a WRP400, is there anyway you can hack a toll bar through router options so that you can restrict toll calls, but still be able to make them when you need to?