Hi Zhang,

The native registration support for Ex90 and lot of other video endpoints has started with CUCM version 8.6 and later.So you can try registering but that won't be a native SIP registration.

You can try below steps for the registration.

1. Create a SIP digest user and associate it with the third party sip.

2. Add SIP third party device endpoint (advance).

3. Set all necessary field such as Device Pool, Location, Line (Partition & CSS), SIP Profile, Device Security Profile.

If the above doesn't works and if you are only looking for calling purpose you can also go with the SIP third party (BASIC).

adittionaly check another thread realted to your problem.

https://supportforums.cisco.com/thread/2083453

Thanks

Alok