Zero-cross triac-output photocouplers are not capable of phase control. Non-zero-cross ones must be used. To perform phase control, instead of a DC current, a pulsed current is passed to the LED, and its pulse width must be made longer than the time until the triac turns ON (turn-on time). In addition, as the current passed to the LED becomes larger, the time until the triac turns ON becomes shorter. Therefore, the pulsed current passed to the LED must be designed by taking into account not only its width, but also its intensity.
Refer to Toshiba Application Note "Triac couplers-basic properties and application design", section 10.2 Triac coupler basic operation by phase control as well.