Call forwarding and Simultaneous Ringing are good solutions when you can know the extensions or numbers where you will be able to accept calls when moving around. But what if you need to move to another office location in a middle of a call and you don't know the number there? This is when you can use the Call Park feature.
With Call Park you put a call on hold in a system-wide "parking area". The call can then be retrieved from any line on the phone system of your company (but not from external phones!)
In practice, when you park a call the system assigns it to a special virtual extension. The system will tell you the number of this extension and you can hang up. You can then retrieve the call from another phone by dialing the relevant virtual extension. Until you retrieve the call, the caller will be hearing music as if on a regular hold session.
It is possible to assign an access code to the parked call in case you want to make sure you are the only person who can retrieve it.
Call Park is also a great feature to increase responsiveness to clients. For example a client calls and asks the recepionist to speak to a specific agent, who happens to be already busy with another call. If the recepionist transfers the call to the agent it may end up in the agent's voicemail (depending on overflow rules for the system) - which some clients find frustrating. Instead the recepionsit can park the call, and notify the agent he has a call waiting for him on the relevant virtual "parked" extension.
A variation on this scenario is useful in warehouses, supermarkets, or other bussinesses where employees are not bound to their extensions or where they share a phone to receive occasional calls. For example a receptionist receives a call in a car shop. She will park it and make an announcement using the speakers system (that also happen to be integrated into the phone system) letting one of the mechanics know they have a call, that he can retrive from the nearest phone next to him by dialling the relevant virtual "parked" extension.