Hi , I have configured kamailio as an IMS server as VM based . PCSCF - 10.x.x.179 ICSCF - 10.x.x.79 SCSCF - 10.x.x.73 HSS - 10.x.x.75 I am using zoiper client for testing the call flow . The *registration is happening properly* in my both zoiper clients for hss configured users (alice and bob). When i am making an *INVITE , *I am getting *403 Forbidden - Domain not served *error from SCSCF. The Call Flow for invite is as follows . call from alice(a)sip.example.com zoiper1 (UE) -> PCSCF ( INVITE sip:bob@sip.example.com;transport=UDP ) PCSCF -> SCSCF ( INVITE sip:bob@sip.example.com;transport=UDP ) SCSCF -> PCSCF (100 trying) SCSCF -> ICSCF ( INVITE sip:bob@sip.example.com;transport=UDP ) ICSCF -> SCSCF (100 trying) ICSCF -> SCSCF ( INVITE sip:bob@sip.example.com;transport=UDP ) SCSCF -> ICSCF (100 trying) SCSCF -> ICSCF (*403 Forbidden - Domain not served*) ICSCF -> SCSCF (Request: ACK sip:bob@sip.example.com | ) ICSCF -> SCSCF ( 403 Forbidden - Domain not served ) SCSCF -> PCSCF ( 403 Forbidden - Domain not served ) PCSCF -> UE ( 403 Forbidden - Domain not served ) Below are the alias configuration i have done in pcscf,icscf,scscf nodes *PCSCF.CFG* # IP-Adress for incoming SIP-Traffic, in the following format: # SIP / UDP listen=udp:10.x.x.179:4060 # SIP / TCP listen=tcp:10.x.x.179:4060 # SIP / TCP/TLS listen=tls:10.x.x.179:4060 # IPSEC / UDP #!define IPSEC_LISTEN_ADDR "0.0.0.0" #!define IPSEC_CLIENT_PORT 5062 #!define IPSEC_SERVER_PORT 5063 #alias="sip.example.com:4060" alias=pcscf.sip.example.com:4060 #!define MY_WS_PORT 80 #!define MY_WSS_PORT 443 #!define PCSCF_URL "sip:pcscf.sip.example.com:4060" #!define TCP_PROCESSES 8 #!subst "/NETWORKNAME/sip.example.com/" #!subst "/HOSTNAME/pcscf.sip.example.com/" *ICSCF.CFG* # SIP / UDP listen=udp:10.x.x.79:4070 # SIP / TCP listen=tcp:10.x.x.79:4070 # SIP / TCP/TLS listen=tls:10.x.x.79:4071 alias="icscf.sip.example.com:4070" #!define NETWORKNAME "sip.example.com" #!define HOSTNAME "icscf.sip.example.com" *SCSCF.CFG* # SIP / UDP listen=udp:10.x.x.73:4080 # SIP / TCP listen=tcp:10.x.x.73:4080 # SIP / TCP/TLS listen=tls:10.x.x.73:4081 #!define NETWORKNAME "sip.example.com" #!define HOSTNAME "scscf.sip.example.com" #!define URI "sip:scscf.sip.example.com:4080" alias=scscf.sip.example.com:4080 # ENUM-Server to query: #!define ENUM_SUFFIX "scscf.sip.example.com." Kindly let me know where i am missing . Kindly need help in this issue since the registration is happening successfully and dropping in the INVITE level. Kindly help.