I take it you have a Symposium Express?
The option you have with Call Treatment is to roll over to another Skillset after the specified number of seconds. In order to get it to work you have to specify the Overflow Skillset in your Call Routing (Priority with an arrow).
However, you want to overflow to an extension? - Try experimenting with Second Overflow Treatment - activate it via On/Off set the 30 seconds and choose "Route calls to this number" set 0.
Second Overflow is ment to be used when you have a special busy day, but you can use it permanently. Remember that it will affect ALL Skillsets, which use the Treatment.