This is missable.
Once you've been able to reach the sawmill in chapter 11, you can get through the gate with code 3324. (This is written down in a locked room that you can access through a red tape in the opposite building.) Then...
As soon as you see that, pull up the number for the VA in your phone book and call right away. If you leave the mill, your signal will drop and you won't be able to make the call.