If you leave your availability/ calendar blank (unknown), company members will be able request an update on your availability.
When another freelancer views your profile, if they aren't connected to you they won't see your calendar and instead will see a note 'Connect with XXXX to check their availability'.
If you are in a staff job, you can select that option whilst marking your calendar as 'unavailable'. This will advise people to get in touch with you to discuss your availability.
TIP: Although it's good to encourage people to contact you directly, a blank calendar can suggest that you're not currently looking for work so wherever possible it's best to add in your currently availability status.