Hello Vijay,
T-CSI and O-CSI are CAMEL triggers for terminating and originating calls respectively.
When MS gets into the roaming area and MAP ( Update Location ) request will be sent from VLR to HLR, the HLR other then MS info also updates VLR with O_CSI triggers.
Where as in case of terminating cals, the GMSC will be updated with the T_CSI embedded in Routing Info_ACK message.
Hope this helps…………
-Thanks
Hari