Also, does the soft phone have to always be on or is it automatic when a call comes in?
The soft phone is only active if the computer is running and the soft phone is running.
The soft phone runs in the background and will be activated when a call is received.
Turning off the computer would be similar to unplugging one of your current phones from the wall. If a call came in for that phone, it wouldn't ring because it's not on. However, the call will be handled according to what the user chooses - e.g. forward to voice mail, another extension, a mobile phone number, etc.