the key Ki from the IMS services identity module (ISIM) contained in the UICC card, calculates locally from the random number (RAND) received, the authentication code AUTN and RES and the keys IK and CK. 11) If the authentication code calculated, AU is identical to that received, the IMS network is authenticated and the Alice's UA entity (or Bob's) sends to the P-CSCF entity a second SIP REGISTER request comprising its identity private, the authentication code RES answer in parameter response of the Authorization header. The REGISTER request, in the header Security-Verify, includes the security data received in the header Security-Server in the response 401 Unauthorized. REGISTER sip:ims.mnc01.mcc208.3gppnetwork.org SIP/2.0 Authorization: Digest username="alice_private@homeA.net", realm="ims.mnc01.mcc208.3gppnetwork.org", nonce=base64 (RAND + AUTN + server specific data), algorithm=AKAv1-MD5, uri="sip:ims.mnc01.mcc208.3gppnetwork.org", , response="6629fae49393a05397450978507c4ef1" Security-Client: ipsec-3gpp; alg=hmac-sha-1-96; spi- - c=23456789; spi-s=12345678; port-c=2468; port-s=1357 Security-Verify ipsec-3gpp; q=0.1; alg=hmac-sha-1-96; spi-c=98765432; spi-s=87654321; port-c=8642; port-s=7531 . . . 12) The P-CSCF entity transfers the message REGISTER to the I-CSCF entity. Basic Procedures 13) I-CSCF entity transmits to the HSS entity the DIAMETER LIR (Location-Information-Request) message to retrieve the IP address of S- CSCF entity. 14) The HSS entity provides the IP address of the S-CSCF entity in the DIAMETER LIA (Location-Information-Answer) message. 15) I-CSCF entity sends to the S-CSCF the REGISTER request. The S-CSCF entity compares the value of the RES received from the UA entity with that of the RES received from the HSS entity. If the two values are identical, the mobile is authenticated. 16) The S-CSCF entity sends to the HSS entity the DIAMETER SAR (Server-Assignment-Request) message to retrieve the mobile profile. 17) The HSS entity transmits the profile of the mobile in a DIAMETER SAA (Server-Ass