Try this (haven't tested this):
use your face down context to set a custom %FACEDOWN variable to 1.
Then add an event and select variable set. Use your %FACEDOWN
variable.
In the action you can change silent mode to on when %FACEDOWN is 1 and
perhaps also add silent mode again and set it to off when %FACEDOWN is
0
This way you can use this one event (that's why I didn't use the value
for the event itself) to turn it on and off (using another context,
e.g. certain caller id).
face down -> variable set to 1 -> fire variable set event -> silent
mode on when variable = 1 (-> continue to 2nd action -> don't turn
silent mode off cause variable != 0)
important caller id -> variable set to 0 -> fire same variable set
event -> do not turn silent mode on cause variable != 1 -> continue to
2nd action -> turn silent mode off when variable = 0
Perhaps this will work.
Hope you can understand it. :D