Hi Fred,
1)A call may drop when you move from a zone of coverage to a zone of no coverage.This is always the case when the mobile subscriber moves across cells that do not overlap to each other so the gap causes the drop since there is no coverage at that point.To solve this problem you can use a Booster to extend the DL coverage of one of the cells to overlap OR you use a repeater to give coverage to that gap.
2)You can also have drop calls in an urban area where the uplink is not matched to the downlink thus causing signals not to be able to reach the BTS since the RX Lev is low.To solve this,networks must always be balanced DL=UL (balanced Network) or you use TMAs to improve on the UL sensitivity of the BTS and also the RX Lev.
If you have any doubts or querry do not hesitate to contact me>you can send me your email ID so that I can send you more on this.
Best Regards
Vuyof