This is probably not how that module was intended to be used.
Try listing each whitelisted number explicitly in the outbound route dial patterns. I don't know if there's a limit there, but if there is I might be able to find patterns in some groups of numbers that would let me reduce the actual number of entries needed. Then just don't provide any outbound route for numbers/patterns that I don't want to enable.
It has been suggested to me that since I am using Trixbox, which has a "forked" version of FreePBX, it isn't certain what might or might not work there. It may even be that Custom Contexts will never work right with that forked version - there isn't really any way of knowing. Why not switch to a distribution (such as AsteriskNOW, Elastix, or PBX in a Flash) that contains true FreePBX?
In the next install, I will :)