I'm trying to integrate Exchange UM with 3CX Phone System. I've just started, so the issues are quite basic. Exchange allows to specify the IP Telephone gateway by IP address only, no authentication options etc. It has to be configured as a SIP trunk on IP PBX. When I point exchange to my 3CX and start the MSExchangeUM service, the following SIP conversation happens: MSExchangeUM logs "unsuccessful ping" error message after that. Any clues on how to configure 3CX so that Exchange can connect and doesn't get 404 right away?