Call flow in gsm

PSTN to Mobile:

1. PSTN will initiate the call and GMSC will looking for the MSRN of called party
PSTN-----[MSISDN]--->GMSC

2. GMSC will forward the called party's MSISDN to get the routing information from concern HLR.
GMSC-----[MSISDN]----->HLR
3. HLR will forward the IMSI to concern MSC to get the MSRN
HLR------[IMSI]-----MSC

4. MSC will send the MSRN back to HLR as acknowledge
MSC-------[MSRN]----->HLR
5. HLR will send the MSRN to GMSC
HLR------[MSRN]----GMSC
6. GSMC will communicate with MSC to set the call immediately
GMSC----[MSRN]----MSC
7. MSC will route the call through BSC , BTS to MS.
MSC---------------->BSC
BSC---------------->BTS
BTS---------------->MS

please let me know if you any more queries.

Md. Isme Azam
Engineer, Core Network Operation
An Orascom Telecom Bangladesh Ltd