Departing an airport in CAS you can get a full departure clearance. This is the standard international situation.
Departing an airport OCAS you get (at most) just a provisional DC e.g. squawk 1234, climb FL050, remain OCAS, on track DVR, contact London Control on 123.45.
Or you get nothing, especially if there is nobody in the tower, or it is a VFR-only airport. In that case you depart and try to fly in the general direction you want to go, while sometimes frantically
making radio calls to the regional IFR control unit to pick up an IFR clearance before you reach CAS or reach the nearest TCU
On the phone... not officially anywhere I know in Europe of but I have heard of it done. It is certainly possible. In the UK, the numbers are unpublished.