The first thing which happens when a Roamer tries to latch onto a vstd network is
1) Send Auth Info message is sent to the HLR.
2) On success of SAI an UL is sent to the HLR. The HLR checks the aunteticity of the VLR(the roaming agreements and stuff) and sends back Insert Subscriber Data which has the Subscriber Info including CSI information, Call Fwd Call Bar etc.
3)The old VLR is sent a cancel Location by the HLR.
4)The Insert subscriber Data(ISD) may contain multiple messages. The VLR sends ISD ack for each one of them.
5)Finally an UL-Ack is sent by the HLR for successful completion of the transaction.
If the HLR finds that the UL has come from a fraud/non partner network it sends a UL reject for the same.