If I effectively cancel my Telkom landline by porting the number to FreshPhone, would this require me leaving the FreshPhone app running on my cellphone 24/7 in order to catch incoming calls ?
The app makes use of push notifications so it will open the app only when the call is pushed to you from their servers.
Does the app run in the background consuming minimal resources ?
Same answer as above.
Can it run on 3 or 4 cellphones simultaneously- one for each family member ?
As I understand, because it is a free service, they do not allow the ability to register a single account on multiple devices, this will result in the registration being taken by each device constantly as it re-registers. They do allow 5 accounts to be created per user so you could register one on each phone, you would just need to set up a quickdial to the other person's freshphone number, this is easy in the app.
How about the possibility of free voice communication between the various connected family members logged into the FreshPhone app as above ?
Yes, their on-net calls are zero rated.