Outside dial tone is an option that allows a route pattern to play a secondary dial tone when it is matched.
You will hear the secondary dial tone as soon as the call routing component selects the best match route pattern and the route pattern has the Outside dial tone checkbox set. When you, the network administrator, dial a number while expecting to hear a secondary dial tone and you don’t hear it, this means that there is at least one overlapping route pattern that matches and that does not have the Outside dial tone check box set.
If you just dialed the access code (for example 9) and immediately heard the secondary dial tone, then you probably have a route pattern like 9.! which has the Outside dial tone checkbox set.
The secondary dial tone sounds different that the internal dial tone that all users hear, when they pick the handset. However, this can be changed with a service parameter tweak. In the Service Parameters page, there is a service called “Always use the internal dial tone”. If set to true, this service forces the system to play a secondary dial tone that is identical to the internal dial tone.
In CUCM, the outside dial tone is simply a checkbox in both the Route Pattern and Translation Pattern configuration pages.