Pradeepgali,
First of all there isn't a way to provide the caller with a Busy Tone, on SIP Trunks if the DID/E&M Recieve Busy instead of Camp-On flag had no effect, unless all of your lines are busy. This is a PBX and its main purpose is to deliver phone calls to users. What you are asking a bunch of technicians and programmers whose sole purpose in life is to make sure those calls go somewhere to get answered is not a normal task and therefore not a simple one.
Secondly, the Camp-On flag that you changed under SIP Trunk Group make sure that is set to YES. This flag is for the devices on the system to be able to camp on to the Trunk Group and wait for a trunk to get out on if they are all busy, not the other way around.
Third, you are using the default keymap which tell most of the technicians/programmers here that your original installing company most likely left a lot of things defaulted, which leaves a lot to the imagination on how your system is really setup.
Now, What do you want to happen to these calls when you are "busy" with the call you are on? The options I will provide you will be nominal, but we may need to know how the calls are getting to you in the first place to find out what is really in charge of that call; my guess is a Hunt Group but I have been known to be wrong.
If you don't know how to answer that then go to System > SIP Peers > SIP Trunk Groups > 92002 > Trunk Group COnfiguration and look for the following two values.
Day Ring-In Type
Night Ring-In Type
Let us know what those are so we can direct you to the next stage and hopefully get this resolved for you.
Thanks,
TE