Error :- System error, please contact administrator
Solution:-
Make sure the configuration is correct.
There is no firewall between the client PC and the call manager.
Update the LMhost and hosts files on the client PC.
It might be due to network connectivity issue between the Call manager and Client PC.
In my case it was a different reason:-
the manager and assistant phones were associated with multiple users.
Removing the multiple associations for the phones solved the issue..