was is working fine for these users at any point of time ?If yes when did it broke ?I mean something must have chnaged.Were there any patches installed on Exchange server ?

Abhishek,Few things to check.-Is the phone subscribed to that service ?-what is the service url ?-does it have a hostname or ip address of the server ?-what happens when you copy paste the url in browser address bar ?~Neeraj

Prem,Looks like you have already checked lot of things just to add to that few more points1.Were these users migrated from some other exchange?2.Have you tried deleting and recreating the users/subscriber in unity ?3. Did you already try the resync m...

what is the ccm/cucm version ?Can you also give a sample call flow ?

see if this helps http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a00805b6f33.shtml~Neeraj