1. create sip extension
2. create sip trunk
3. create outbound route
4. install custom context, download from http://mirror.freepbx.org/modules/release/contributed_modules/
5. create custom context using this outbound route
6. set ext. use this context
beware after install this conetext customcontexts-2.8.0rc1.1.tgz, make the Priority
local-ext back to 50 from 96, otherwise local ext. cannot be ring each other