Avaya and Exchange UM integration - something you need to know about Exchange

https://johanveldhuis.nl/exchange-um-accepteerd-geen-oproepen-meer-na-de-upgrade-naar-sp1/

https://social.technet.microsoft.com/Forums/exchange/en-US/a156daf9-7793-43b6-bbb6-3bd282d5cf7a/um-2013-does-not-answer-calls?forum=exchangesvrunifiedmessaging

Exchange Server runs two unified messaging services, umservice.exe (on Exchange 2010 and Exchange Server 2013 Mailbox Servers) or Microsoft.Exchange.UM.CallRouter.exe (on Exchange Server 2013 Client Access Servers) that listens on TCP 5060 and UMWorkerProcess.exe (both versions of Exchange Server) that listens on TCP 5065 or TCP 5067. The correct process for connecting to Exchange Server unified messaging is to connect to TCP port 5060 and get back a SIP Redirect to either port TCP 5065 or TCP 5067. The reason for the redirect is that Exchange Server starts listening on 5065 and after a week starts a second process listening on 5067 and once the process on 5065 has finished all its call handling it will stop the process listening on 5065. This way Exchange Server manages the process, memory management, etc. without needing to restart the process if it goes bad – it just starts a process on the other port from the current process and directs all new calls at the new process.