Usage of NSF setting in zapata.conf

am newbie to ISDN. Am having a doubt in National Specific Information Inof Element setting.
How exactly it works?
1.If we set NSF as Toll Free Megacom, will it effect an Incoming call on ISDN?
2.Suppose we set it as SDN, then if we want to make an Outgoing call, which is a toll free number, then what happens?
3.Is it fix or changes according to our requirement?

Plz help me…

Thanks in Advance