SIP/2.0 503 Service Unavailable
Error Message: Cannot sign in because the server is temporarily unavailable. If the problems persist, contact your system administrator.
Issue: Large amount of Client are unable to login on communicator and getting above error message.
When I checked
Here are the best practices to resolve issue.
1. Ask for Login / Logout from MOC or Lync Client.
2. Do to flush DNS (Ipconfig /flushdns).
3. Check critical alerts generated by server for better understanding of issue.
4. Service control manager gives alerts.
5. Here I did all above and later point of Enabled logging and reviewed logs.
7. Find out the affected Front End server where all affected client to connect.
8. Restarted service one affected Front End server.
9. After restart client resume and properly connected again.