Ground to ramp/apron can be via telephone or something, too.. if I remember some "best of JFK ATC" correctly.
Long story short: it's complicated ;-) Most - if not all - ATC recordings I've been listening too had a handover (incl frequency) to the next controller (whoever it be, ATC or X) anyway. Eg. at or before being at the holding point, ground will "speedbird 123, contact twr at 131.25 (for departure/clearance)".