The diac between the gate and MT2.
David,
it works fine, because on-hook position no current-flow (hookswitch off),and line voltage about 50V.
If the hookswitch on, the DIAC fired the TRIAC, current can flow, and the line voltage decreased about 10V. (DIAC can conduct, when the voltage of pins about 30V) The parallel phone also contain same circuit, and these circuit can't work with 10V.(My experience: if i switch off only one wire of phone, in full silent i can hear some conversation in parallel device.)