Hello Nitin,
Got the same issue one year ago, from B7 to B8. It was a missing patch on the NSS side, leading to many TCH assign fali due to prep BSS, especially during the peaks of traffic.
The problem was real, and not just a counter overestimation. Subscribers couldn’t setup calls… Maybe you should do some drive tests in order to validate that the problem is real or not.
Otherwise, it might be related to the TCH Queuing, Directed Retry and Forced Directed Retry.. check all those indicators as well. I observed that the issue was happening when the tch queue was highly used.
Good luck with all this… don’t expect your vendor (alcatel ?) to help you with this issue, because they have no answer for you 😀 Check the NSS patch…