Reason for SDCCH and TCH can be different in different network. Mainly reasons for TCH are high Timing advance, Bad quality of radio network, Bad link quality, interference, congestion and wrong mapping of PCM.
TA drops happen if cell is shoting more than 32 km , so the solution depends on the coverge objective .
If you really want to cover that far like hwy site then you need to set the cell to extended range .Of course everythng comes with a price so if you turnon extended range your capacity will cut in half
IF the cell objective is less than 32 km then you should cut the coverge by tilting or/and by reducing the power