You have to think about it this way - the FXO port on the router is just like a plan analog telephone. When you pick up a normal analog phone, you have information about what the number of that phone is. There isn't any method of information passing to send this information to the phone, nor is there a need to.
Because of this, the gateway will not automatically know where to send the call. This is what the connection plar statement is for, so that it sends it to a specified DN. So in this case, 100 would either be the auto attendant or DN of a user that would take the calls.
hth,
nick
