I set my extensions_custom.conf thusly:
[from-trunk]
;retrieve RDNIS
exten => _XXXX,1,Macro(stdexten,${CALLERID(rdnis)},${CALLERID(rdnis)})
exten => _XXXXX,1,Macro(stdexten,${CALLERID(rdnis)},${CALLERID(rdnis)})
exten => _12599,1,VoiceMailMain()
exten => _12599,2,NoOp(${Mailbox})
exten =>...