Hi, Afaik, this is a forwarding from ORIGiniating S-CSCF to TERMinating I/S-CSCF. If the terminating user is registered at another S-CSCF or even at another domain than the originating user, then this procedure is obviously meaningful. If we have only one network and one S-CSCF, then the procedure seems to be redundant, however for clarity of call flows, the procedure is not optimized but kept as it is. If you have only one (logical) S-CSCF in your network, then you could try to omit the I-CSCF, just an idea. Hope this helps. Christoph From: sr-users <sr-users-bounces(a)lists.kamailio.org> On Behalf Of Iman Mohammadi Sent: Thursday, May 23, 2019 12:26 PM To: Kamailio (SER) - Users Mailing List <sr-users(a)lists.kamailio.org> Subject: [SR-Users] Call Routing at IMS Hi Everyone When P-CSCF receives INVITE from client it sends INVITE to S-CSCF then S-CSCF without any look up(checking the user is registered or not)sends INVITE to I-CSCF then I-CSCF sends LIR and receives LIA and sends INVITE to same S-CSCF (in case of one S-CSCF exists).My Question is here why without any look up S-CSCF sends INVITE to I-CSCF??this procedure affects system performance and increases call set up time The information contained in this e-mail message is privileged and confidential and is for the exclusive use of the addressee. The person who receives this message and who is not the addressee, one of his employees or an agent entitled to hand it over to the addressee, is informed that he may not use, disclose or reproduce the contents thereof, and is kindly asked to notify the sender and delete the e-mail immediately.