Almost always registrations as call signs, sometimes preceded by Court Line e.g. "court line golf mike golf". Best one I remember was when the TrStars had US crew, training
"Luton Approach TriStrar Trainer Triple Alpha"
Flight numbers were used for a while in I think 1970- I think because it was easier during some kind of European ATC dispute. Britannia changed at this time too and never reverted to registration callsigns but Court Line did and kept them to the end.