I’m sorry if my question is stupid, but what do you mean by “terrestrial” ?
Are you getting 4% congestion rate on the radio channels ? In this case, you should be checking the number of channels on the radio interface, not on the terrestrial interface.
Final question : are you measuring TCH congestion or SDCCH congestion ? 0.92 erlangs seem a very low amount of traffic for TCH, especially on a cell with 58 ts for TCH…
IF during 1 hour you’re getting 4% TCH congestion rate on a cell with 58 TCH timeslot, and getting an hourly TCH erlang of 0.92 Erl, then it is either :
1. the software that is “freezing” the timeslots (cf. Naren’s post)
2. you TCH timeslot are preempted by PDCH (GPRS traffic), or SDCCH (signalling traffic, if your TS are defined as dynamic TCH/SDCCH). Therefore, check your SDCCH traffic and GPRS traffic (or PDCH occupancy)..