How to suspend an activity while waiting for an external decision ?

Hello,

I'd like to know if it's possible and how to do when :
- you began to work about an activity
- you encounter a problem needing a decision from an external actor - and you don't have any clue when he will take this decision
- so you want planning calculation to not schedule the activity any more (hopefully not endlessly

I've tried to put a milestone in the future and a dependency to the activity, but as far as I've seen it doesn't impact the activity scheduling when there's already some work done.

Any way to do this or any hint ?

BTW, kudos for all this good work !

Thanks,

Robert