Unlike call forwarding where an incoming call will bypass your office phone, simultaneous ring is having inbound calls ring to your office phone and up to two additional phone devices at the same time. Hence the term, “simultaneous ring”.
For people extensively using their work phone while at their desk, but who also frequently move about carrying a mobile phone, this is a practical option to consider to avoid missing incoming calls. Also, if you are not effectively changing the state of your Mitel phone extension (you are always “available”) then every time you move about this feature can be a great convenience for you.
The instructions attached to this article illustrate how to setup your phone's Simultaneous Ring feature.