I found this in the online help:
To provide for call restriction after CO dial tone (Optional):
1. ARS Digits Dialed form:
Digits Dialed = blank
Number of Digits to Follow = unknown
Termination - route
2. ARS Leading Digits form:
Second Dial Tone = Alternate
But in the ARS digits dialed form we can't insert blank (valid are 0-9,#,*,N,n,X,x,Y,y)