In H323Transport::HandleFirstSignallingChannelPDU, if NULL connection is returned from endpoint.OnIncomingConnection, the code try to send a release complete to the other side. However, line 1292 is not going to work (the connection is NULL): releaseComplete.Write(*this,*connection); To fix this, I added a Write method which does not accept a connection. Note that the new method duplicate the code from the version that accept a connection - I did not have to time to do it properly. The patch is created against version 1.20.2-alpha2. I verifyed that the issue still exists in the trunk. http://h323plus.cvs.sourceforge.net/h323plus/h323plus/src/transports.cxx?vie...