Login question about from another Domain with out using DNS

Hi All

I have 1 Lync 2010 Standard and Edge, both on 192.168.96.x network.

some machine cannot using our DNS, but they would like to use, the Lync Client,

In their host table,

I put following record

192.168.96.2sip.a.domain.com

192.168.96.2LyncFrontEnd.domain.local

192.168.96.3CA.domain.local

In the Lync Client, Manual Configuration , put the internal server name: LyncFrontEnd.domain.local:443

in the client, I can telent sip.a.domain.com 443 and 5061

and and the root CA is already trust

but when I try to login, the error "Cannot sign in because the server is temporarily unavailable. if the problem continuses, please contact your support team."

Any ideas? Thanks