Autodiscover seems to be working fine with everything else.

When I run get-autodiscovervirtualdirectory it comes back blank for internal and external url.

I have read these can be blank, but do they need to have fields inserted for the CX600 to work?

The CX600 can be picky about this, but it's really an Exchange issue.  Your autodiscover urls need to be set for the phone to locate EWS properly.  If you right click the Lync icon in the system tray and go to configuration settings, are you seeing EWS urls in there?

Hi monolithickernal,

Have you set up the SRV record for Exchange Autodiscover ?

This issue is a bit odd as it does not seem to be a definitive requirement, only occurs in some deployments Exchange integration has failed on all phones until the _autodiscover._tcp.contoso.com DNS Service Locator (SRV) Record was created.

Best regards,

Eric

Yes, EWS is setup and SRV record for Autodiscover is also setup.

What is the CU version running on the device?. I had the same issue with a device running on an older firmware. Updating it to the latest CU fixed the problem 

Version: 4.0.7577.4372

Updated to Version: 4.0.7577.4455 and Calendar started working.

It is only showing Lync Meetings.  It makes since, but are other meetings suppose to show up?