By default, when all agents are busy, we send the caller to the waiting queue. However, it is possible to set a backup number (If-No-Answer phone number) that we ring when no other agents are available (we send the call to this number before it is sent to voicemail).
In order to send the call to the backup number instead of the waiting queue, you have to disable the waiting queue. You can do it by going to Admin - Preferences, and setting the "Maximum Queue Size" to "Disabled”:
Go to the desired phone number settings (Admin - Numbers, then click on the phone number) and set the "If-No-Answer Phone Number” to be your office phone or desired external line.