Well, the reason I ask for more information is that changing the camp on timer for the trunks can adversely affect other functionality on the system. It may fix your immediate problem but cause others that have more impact.
Not knowing why you want to do this limits the options for solutions.
A typical example of your scenario is fax machines. There are other ways to resolve the issue of providing busy tone for fax machines. I would even say they might be prefered.
But I'll go with your short answer. "Raasoning, client simply wants extensions to get busy tone, that's it."
I can't help people that won't help themselves.
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What's most important is that you realise ... There is no spoon.