Lync clientonWindows XP Sp3doesn’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