What are the possible causes for a Drop Call on a UMTS network?

What are the possible causes for a Drop Call on a UMTS network?

             Poor Coverage (DL / UL)

 Pilot Pollution / Pilot Spillover

 Missing Neighbor

 SC Collisions

 Delayed Handovers

 No resource availability (Congestion) for Hand in

 Loss of Synchronization

 Fast Fading

 Delayed IRAT Triggers

 Hardware Issues

 External Interference