congestion is not the only cause of call setup. it can be due to faulty hardware equipment, or more probably to radio problems (poor coverage, strong interferences)
keep in mind that a MS in call setup will jump from BCCH to SDCCH to TCH. So the MS is experiencing a lot of different frequencies, different timeslots, different TRX’s. If one of them is faulty or interfered, the call setup will fail.