Jerrys,
in radio, there are mny possible causes for one problem. For instance a SDCCH drop can be caused to interference, low coverage (uplink, downlink), by wrong settings of radio parameters, wrong settings of nss parameters, or even a BTS failure or an Abis failure.
The impact on subscriber is simple : he will feel like the call setup was not completed. He pushed the green button, but the call failed in the first few seconds.