I discovered this by accident setting up Google 411 , perhaps nothing new to you, but here it goes;
1- Create an ENUM trunk. call it whatever you want
2- Create an Outbound Route, call it whatever you want
Paste this to the Dial Patterns of your Outbound Route
1800NXXXXXX
1822NXXXXXX
1833NXXXXXX
1844NXXXXXX
1855NXXXXXX
1866NXXXXXX
1877NXXXXXX
1888NXXXXXX
Then set the Trunk sequence to your
ENUM trunk on top.
To verify if the 800 call is going out with your ENUM trunk
Run this from the console asterisk -rvvv asterisk
Enjoy.