When the mobile is idle, it listens to the best cell to camp. Mobile decides to choose the cell by itself without the help of BSC. This is done by comparing signal strength of each BCCH frequency and if found the strongest then it will camp once the C1 > 0 (Cell Selection Criterion).
If after camping to this cell, it finds that a neighboring cell is much better then it will change to that neighboring cell. If the new cell is in same location area the mobile does not have to inform the BSC about its new cell but if the new cell is from a different different location area then the mobile will perform a location updating to inform the BSC.
A mobile is considered busy when there is a call going on (speech, data or fax) or it is in the middle of a call setup. At this stage the mobile cannot decide by itself whether it is necessary for the mobile to handover to a better cell.
Only BSC can determine if a mobile has to change to another cell other than the serving cell. BSC makes the decision based on measurement reports sent by both Mobile and BTS. This decision making is called locating. In a busy state, mobiles can receive Short Text Message (SMS) but cannot receive Cell Broadcast Messages.