Lync front end server and Certificate server is unavailable and it is on remote site. 

SBA ,  DHCP and DNS server and DHCP options is ok. SIP serve point to SBA, certificate server point to Remote site server .

Only Lync client soft phone can login , cx600 keep on connecting to lync server and can't login.

After the Certificate server and front end server the cx600 resume normal. 

Any idea if why cx600 can't login but lync client can?

Lync server version 2010.

Thanks.

Have you installed the latest update for the CX600 4.0.7577.4463?

https://www.microsoft.com/en-us/download/details.aspx?id=23866

You can activate the log on the phone to check, why the phone has problems to connect.

No , it is old version.

4.0.7577.4372.

Does it relate to the firmware problem?

Hi Jo Tse,

You may try the following suggestions.

First, its recommend to install the latest firmware for the device.

Second, please check if theres something on network which blocks the network connections to the Center site. (e.g. firewall)

Third, refer to the Jeffs article and double check your configurations.

http://blog.schertz.name/2010/12/configuring-lync-server-for-phone-edition-devices

Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.

Best regards,

Eric