Ran into this problem today when trying to sign into an older Polycom CX700 phone on Lync 2010. The phone kept complaining that it couldn’t find the domain and download the certificate even though all other phones worked absolutely fine. Normal syntax for entering credentials in Windows is either Domain\Username or username@domain.localThe trick to logging in is to the CX700 is to use domain.local\usernameThis should now enable you to sign in to the device unhindered!