check the usage (TCH traffic erlang) per TRX, and check your hardware is “available”. Maybe some of your TRX’s are down –> a cell with 4 TRX has a capacity of 21 erlangs, but if only 1 TRX is available, with 3 erlangs of carried traffic, you’ll have a high congestion rate.
Also, check your GPRS traffic.. the gprs might be using some of the TS, reducing the voice capacity of the cell.
low usage, high blocking : before deploying more TRX’s you must ensure you are using your existing TRX as much as you could.
Can you give us an example of traffic erlang at busy hour for one cell (and what’s the config of this cell)