There is an ongoing session between UE A and UE B.
Step 1.
An incoming INVITE is received at the S-CSCF of UE A.
Step 2-6.
Via Initial Filter Criteria the INVITE is forwarded to the TAS and the SCC AS, before it is forwarded to the MGCF.
Step 7.
The MGCF sends an IAM message to the MSC Server to inform it of the incoming call.
Step 8.
The MSC Server sends a Setup message to UE A to inform about the incoming call. Depending on CW settings the MSC Server may send a REL response immediately.
Step 9.
UE A responds with a Release Complete message with cause #17 "user busy".
Step 10.
The MSC Server maps the Release Complete message to a REL message.
Step 11.
The MGCF maps the REL message to a SIP Busy message.
Step 12-14.
The SIP Busy message is forwarded via SCC AS and S-CSCF to the TAS.
Step 15.
The TAS adds a CC possible indicator to the Busy response.
Step 16.
The Busy response is forwarded towards UE C.
Step 17.
A CC Invocation Request is received at the S-CSCF of UE A.
Step 18.
The CC Invocation Request is forwarded to the TAS.
Step 19.
The TAS accepts the CC Invocation Request, sends a CC Invocation Response towards UE C and begins supervising UE A for becoming not busy.
Step 20.
The CC Invocation Response is forwarded by the S-CSCF towards UE C.
Step 21.
The Session between UE A and UE B terminates.
Step 22.
After an appropriate time to allow UE A to initiate a call, the TAS sends a CC Ready notification towards UE C.
Step 23.
The CC ready notification is forwarded by the S-CSCF towards UE C.
Step 24.
An INVITE Request resulting from the CC ready notification is received by the S-CSCF.
Step 25.
The INVITE Request is received by the TAS.
Step 26.
After appropriate checking that the INVITE request is resulting from the CC ready notification the TAS forwards the INVITE request to the S-CSCF.
Step 27-31.
The call set-up proceeds in accordance with normal procedures.