asterisk users - Oct 2006 - set outgoing msn on chan_misdn

hi, does anyone know if it is possible to set the outgoing msn number with 
chan_misdn (the number the people on the other side will see as the caller)

I already tried
Set(CALLERID(num)=1234)
SetVar(CALLERIDNUM=1234)
Set(CALLERID(name)=1234[|a])
Set(CALLERID(number)=1234)

but none of them seem to work :(

Mark