Hello, I'm kind of having the same problem as described in this message: http://lists.iptel.org/pipermail/serusers/2004-September/011452.html When cancelling an INVITE for which only a 100 trying response is received, the CANCEL request does not seem to be forwarded by SER (version 0.8.12). client A SER client B -> INVITE -> -> INVITE -> <- 100 trying <- 100 trying -> CANCEL -> <- 180 ringing <- 200 ok -- no more pending branches <- 487 Request cancelled -> ACK -> <- 183 Session progress <- 183 Session progress <- 183 Session progress <-183 Session progress ... To client A it seems that the INVITE is cancelled, but SER does not forward the CANCEL request to client B, so client B thinks the INVITE is still pending and keeps sending 183 responses (which are received by client A). I'm not sure when SER receives the 100 trying and 180 ringing (before or after the cancel) but the 180 response is not received by client A. If client A sends a CANCEL request after receiving a 180 response, the cancel handling is done correctly. kind regards, Sigrid Thijs