Class Charlie radio procedures: two-way contact to enter
To enter
Two-way radio communication established (the controller uses your call sign).
What "two-way contact" means
You are cleared to enter once the controller answers with your call sign. "Cessna One Two Three Four Five, standby" counts as two-way contact — but a generic "aircraft calling, standby" does not. You also need a Mode C transponder.
The call
"Socal Approach, Cessna One Two Three Four Five, one five miles north, three thousand five hundred, inbound for landing." When they reply using your call sign, you may enter.
No explicit clearance needed
Unlike Class B, you do not need the words "cleared to enter." Establishing two-way contact is the gate.