Hi, If topos is activated and kamailio responds with a 407 "proxy authentication required" to an initial INVITE, INVITE received after with credentials (MD5 response in Proxy-Authorization header) is badly managed. Credentials are never validated and Kamailio responds 407 again. If topoh is used instead topos all works fine and credentials are validated. call flow is: Phone sends INVITE to kamailio SBC kamailio respond 407 "proxy authentication required" because SBC wants to authenticate caller Phone resends INVITE with Proxy-Authorization header with all valid information kamailio responds 407 again instead forwarding INVITE. My code: if (!pv_auth_check("$fd", "$sht(auth_cache=>$var(key))", "0", "1")) { auth_challenge("$fd", “1”); exit; } # user authenticated - remove auth header consume_credentials(); So if topos is used, pv_auth_check always returns false even if the phone sets valid information. If topoh is activated, pv_auth_check always returns true (same phone and same kamailio SBC versioning) I made my tests with kamailio 5.4.3 on centos 7. Regards, Frédéric Gaisnon