Communicator 2007 Automatic Sign-in Failure

In my case this was caused by lack of DNS entries.
Just creating a sip CNAME to my OCS 2007 server was enough, but there are others you can create.

I found a tool that helps you troubleshoot DNS issues here.

A\CNAME records:
  sip.domain.com
  sipinternal.domain.com
  sipexternal.domain.com

SRV Records:
  _sipinternaltls._tcp.domain,com
  _sipinternal._tcp.domain.com
  _sip._tls.domain.com