The phantom abandon call timer is used to classify calls the queue to the splits or skills, and are delivered to agents even when the calling party has disconnected.
It assumes that any call which was answered by an agent, and lasts between 1 and 10 seconds, must have abandoned and was a phantom call.
The timer cannot be set beyond 10 seconds.
The timer setting is created when the cms install script is run. It's not something you can change easily, and would require that you re-start your CMS to implement. Check the file named /cms/install/cms_install/cms.inst.skl for the parameters that would need to be set to re-create your installation file.
The timer is ONLY necessary when your local Central Office does NOT provide disconnect supervision. If you use T1's to carry your call center traffic, you probably do not need the phantom abandon timer setting. Disconnects across T1's generally take less than one second to complete.
If you are simply trying to ignore calls below 20 seconds in length, I suggest that you look into re-defining your call profiles. That is something that can easily be done and would provide you the same information.
Carpe dialem! (Seize the line!)