Yesterday a student and I filed IFR from SFM (uncontrolled). We picked up our clearance on the ground from Portland Clearance, and he said to "call when airborne." I thought this was unusual for a clearance frequency, and when we called him at 500' AGL, he just reiterated the departure frequency from our clearance. Should I have assumed he meant "call departure when airborne"? Why would he have to say that anyway?