You need to change your trunk context to something like from-dahdi-announcement
and then in extensions_custom.conf:
[from-dahdi-announcement]
exten => _.,1,Answer()
exten => _.,n,Wait(1)
exten => _.,n,Playback(this-call-may-be-monitored-or-recorded)
exten => _.,n,Goto(from-dahdi,${EXTEN},1) ; sub whatever context you want in place of from-dahdi