There are only a few simple steps to change the active role in Zac.
At the top of the window you will see your name next to a gear icon
Clicking the gear icon shows you what queues you have available to you
If you click on where the role name is, you can change your active role to the specified group
The number that will display on an outgoing call will reflect what role you are actively logged into. You can only have one (1) role active at one time. You will still receive calls from each role you are logged into but will only display the call-out number of your active role.