Call handling is one of Avaya Cloud Office functionality that will allow you to manage your phone calls. From the web portal https://login.cloudoffice.avaya.com/.
You will find it under Settings then Call Handling.
To add a coworker:
NOTE: Additional pop up window may appear if there are multiple devices are assigned to the user. Select one and click Select.
NOTE: You can change the ring order (see previous notes regarding ring all at one vs ring in order)