Ring Group
In a ring group, you can have calls ring multiple user phones at one time. The first user to pick up the call gets the call and all other phone stop ringing.
In the administrative portal, you have the option to add or delete users at any time, set the amount of time to ring all phones and set a failover location for the call if nobody answers the call. On fail over, you can route the call anywhere (user, voicemail, extension, etc.) in your OnSIP account.