https://kb.mit.edu/confluence/display/istcontrib/Zoom+Phone+Service+-+Forwarding+and+Do+Not+Disturb
use this for when an employee leaves
Visit mit.zoom.us and input \[number you wish to administer\] under "Enter the telephone number to administer" and click "Administer phone". Once in, select phone on the left hand pane to access the phone settings for the phone number you want to administer. Scroll down to the call handling section and following the instructions in the section I linked below.