Sorry forgot to mention the Rach unsuccessful and SD Blocking might be the issue.
There should be TRUNK utalization stats which show if there is congestion on Ater / N7 signalling( BSC To MSC Signaling). But if this is the case there might be CSSR degradation on most cells of the BSC arround bust hour dropping with increase in traffic. If you have any stat like MSC Assign Attempt Rate (Ratio of Channel Required to Assignment Request) it should also drop. Else either the congestion is on SDCCH / TCH both can be checked on stats. or your RACH might not be successful based on UL Interference / Bas BCCH planning. By the way a Drive Test can also reflect if the issue is of RACH unsuccessful / SDCCH congestion/ TCH Congestion.