The problem is the following: when K-S (Kamailio Server) replies it relays the responses to the K-P (Kamailio Proxy) instead of the LB-P (load-balancer of the proxies). I tried almost every solution: changing aliases, adding Path headers, changing destination-URI, changing Via: headers ... But the result is always the same: using transactions it seems that, even if requests are correctly load-balanced, the replies are not relayed back following the same path (in particular, referring to the precedent scheme, a reply for the previous request will follow the path C <- K-P <- K-S).