Originally Posted by
MarcK
Arriving traffic contacts ATC when on the ground, or if cancelling IFR. Departing traffic is given a clearance by radio (possibly a remote outlet) or telephone and a "clearance void" time, typically 10 or 15 minutes later. They contact ATC by radio after departure.
Or the flights just go VFR if the weather is suitable.
For example if an aircarft is on approach and is certain to be able to reach it in VFR they often cancel IFR.
Similarly they would depart VFR and pick up their IFR in the air while they are already in radar coverage.
This allows much more throughput depending on the weather.