It's a standard feature of Phonepower. Go into the
Call Forwarding >
Forwarding Rules section of your CP. In the first (built-in) rule, set a pickup time shorter than the diversion to GV VM (6 seconds is minimum); in the second (last) rule, select the radio button for "This Number" and then select "Busy signal" from the drop-down.
If I'm not mistaken, this will cause incoming calls to go to voice mail when ring time exceeds the value you set in the first rule. If you already on that line, the second rule should send it to "busy" (I think
).