corhavelaar

Try this:

System -> Telephone TAB :
Dial delay time : something between 2000 and 3000 Ms
Dial Delay count : empty

Shortcodes :
Shortcode : 9
Telephonenumber : .
Linegroup : your linegroup number
Function : SecondaryDialTone

Shortcode : [9]N;
Telephonenumber : N
linegroup : your linegroup number
Function : Dial