Meu provedor de VOIP solicitou que eu adicionasse isso em /etc/asterisk/extensions.conf:
exten => h,1,Hangup
exten => _00X.,1, Answer
exten => _00X.,n, Set(CALLERID(num)=96354205144239) ;
exten => _00X.,n, Dial(SIP/${EXTEN}@Trunk_...
18.07.2012 / 09:20