hello robin cooker,
to dimension a network, you actually need to know the busy hour traffic in erlang per cell.
(not the total value for the whole network)
once you have the traffic per cell, you can start anlyse how much traffic channels are needed per cell to carry all this traffic.
the method to do this is to use the Erlang B table, choose a blocking rate of 2% and look up the traffic per cell (offered traffic). It will ouput the number of channels needed.
Divide the nb of channels (= number of timeslots) by 8. Round up the result. This number is the number of TRX needed per cell.