6 TRX = 42 timeslots FR or 84 timeslots HR
42 channels –> 32.8 erlangs capacity @ 2% congestion
84 channels –> 72.5 erlangs capacity @ 2% congestion
If you’re seeing 75 erlangs, it means that most of your traffic is probably in half rate (AMR or not), and that your congestion rate is around 2%.
The erlang B is not exact in GSM, because of the TCH queue…