At times you might experience a strange kind of login issue. Most of the users can login but one or few users can't. Most of the times you face this issue due to SIP related AD attributes issue.
When problem users would try to login in MOC, they will get following error.
"Cannot sign in to Communicator because this sign-in address was not found. Please verify the sign-in address and try again. If problem persists, contact your system administrator."

Resolution:

We disabled the problem user for LCS or OCS.
Did Reset all msRTC attributes to <not set>
Re-enabled user for LCS or OCS.

Followings are the list of attributes we reset:
msRTCSIP-PrimaryUserAddress
msRTCSIP-PrimaryUserAddress
msRTCSIP-PrimaryHomeServer
msRTCSIP-HomeServer
msRTCSIP-HomeServerString
msRTCSIP-TargetHomeServer
msRTCSIP-UserEnabled

Re-enabling the users for LCS or OCS should allow them to login.