IMS Registration signalling flow —
User not registered
R1-3d. SIP REGISTER request, with credentials: between I-CSCF1 and HSS(16 of 22)
I-CSCF1 requests information related to the Subscriber registration status by sending the private user
identity, public user identity and visited domain name to the HSS. The HSS returns the S-CSCF name
which was previously selected in step 1d.