I’ve spoken to a few of my contacts and the advice is to “please contact your channel partner to raise a ticket with Product Support, and they will look into the issue and if necessary get the Mitel design team involved”. Sorry I can’t be of more help, but this will required further technical investigation.
I’ve never heard of anything like this on an IP set before. It sounds like the call isn’t going to the user’s main line and they need to press the “ringing” line, but not if the caller is hearing them answer. Very strange.
I agree with Denise that this is something that may require having a support ticket opened.