Have a client with a Cisco 2921 Router running latest version of CME 9.1 (currently)
Had a weird issue with Call Transfer and the call flow would go like this:
Ext 1001 would dial a external number 0800111222 and then transfer the call to Ext 1002.
The Call then dropped and disconnected after a few seconds.
A incoming call received by Ext 1001 from 0800 111 222 then transferred to Ext 1002 works fine.
So incoming calls works but outbound based calls don’t.
Running “debug ccsip messsages” show an UPDATE message is sent when the call is transferred to the SIP provider and because this was replied to it caused the call to drop.
Adding “no update-callerid” to voice service voip under sip resolved the issue (took days to diagnose and fix this silly issue!)