A small block of custom dialplan will do this, in it’s crudest form it will look like the below. Background reading: Hooking for fun and income
[macro-dialout-trunk-predial-hook]
exten => s,1,Noop(Entering user defined context macro-dialout-trunk-predial-hook)
exten => s,n,GoSub(func-set-sipheader,s,1(Diversion,sip:[email protected]\;privacy=off\;reason=no-answer\;counter=1\;screen=no))
exten => s,n,MacroExit()
You will prob need to substitute a channel variable in place of 911234567