hi,
Many call drops in U900 is usually caused by users getting too low coverage of u900, without possibility of handover to another cell.
check the neighbour relation with GSM cells, check the event 2d thresholds, the 3a threshold.
Also check 3G neighbours…
but really, the first step would be to investigate the kpis and the topology, and understand what is causing the drops.
Cheers
pix