Have pushed on not very far it seems with testing of SER using Windows Messenger v5.0 (on windows 2000) as UA have modded the default ser.cfg with an alias statement to reflect the domain name of the sip clients. this has allowed me to get through the process of getting to sip clients to register with the sip proxy server this is apparently confirmed by status codes of 200 reported in the SIP register messages. i fall at the next hurdle of using WM 5.0 to send an instant message (same applies to file tranfer) to a contact WM reports "The following message <message text> could not be delivered to all recipients" have captured the data from the client which i now summarise; 1. SIP message - INVITE (recipient of the message) to the sip proxy the first response is a SIP status code 100 (which i assume to be some sort of comfort signal) followed by status code 200 - which i assume to be the success response 2. followed by a BYE message from the proxy server to the client and a corresponding ACK from the client to the proxy server 3. SIP / SDP MESSAGE from the client to the proxy server note - the decode of the SDP data section reports "Invalid Line" - followed by text of the Instant message don't know if this is limitation of the Ethereal decoding ?? this is somewhat surprisingly followed by a message with Status Code 200 from client to proxy 4. final death knell of the communication would seem to be a SIP message from proxy to the client with a status code 481 - decoded as "call leg / transaction does not exist" What I am conscious of is that the source / target sip recipients &quot;user1(a)mydomain.com and user2(a)mydomain.com&quot; have not been defined to the SIP proxy i have taken p28 of the SER admin guide to be good to its word that "default configuration script allows users to register with each other" or have i taken this a bit too literal and that some user setup needs to be done my initial thought was that the sip register to the proxy server would create some sort of dynamic entry in what i think is termed as the user contact table in SER. don't know if i am experiencing vagaries of the Microsft messenger UA - would be glad to know of any other "soft" SIP UA applications apologies for the essay - hope the mailing list can be of help here . GT