Там же все написано
> This is the Custom Destination to be published. It should be formatted exactly as you would put it in a goto statement, with context, exten, priority all included. An example might look like:
mycustom-app,s,1
Заходим в /etc/asterisk/extensions_custom.conf и пишем свой контекст
[custom-dial-trunk]
exten => s,1,Noop(----Custom-Dial-Trunk---)
same => n,Set(MY_EXTEN=77777777)
same => n,Set(MY_TRUNK=77777777)
same => n,Dial(SIP/${MY_EXTEN}@${MY_TRUNK},60,rTt)
same => n,HangUP()
Потом в Custom Destination
Description = 123
Target = custom-dial-trunk,s,1