Lync client on Windows XP Sp3 doesn’t authenticate with Lync server Edge (Remote users). Windows 7 able to support NTLM Authenticate but Windows XP does not support 128-Bit encryption. So that we will do following. You have to remove Mark "Require 128-bit Encryption". Guide: Run secpol.msc on Lync Front End server. Select Local Policies then Security Options Set the following values to "No Minimum." Network Security: Minimum session security for NTLM SSP based (including secure RPC Network Security: Minimum session security for NTLM SSP based (including secure RPC) servers Supported Threads: http://social.technet.microsoft.com/Forums/en-US/ocsclients/thread/7dc73306-c365-4785-b712-6e9d38664d1a