Hi Tally,
In CAP, OCSI and TCSI mark are used for CAMEL subscribers. When LU, the OCSI and TCSI mark stall in VLR data base. When subscriber make a call, IDP(initial DP) was triggering directly to IN servers, and MSC/VLR is not intelligent enaugh to now that IN server is not available or not.
One case, When PPS system is not available, here SS7 connection between MSC/VLR and IN servers also not available. When this happen the conversation between IN and MSC/VLR also not available, So call will be rejected.