Error events occur when you disconnect a computer that is running Communicator 2007 R2 from the network: "Event ID: 3" or "Event ID: 7"

Log Name:      Application
Source:        Communicator
Event ID:      3
Task Category: None
Level:         Error
Keywords:      Classic
User:          N/A
Description:
Communicator was unable to resolve the DNS hostname of the login server OCS_Server_FQDN.

Resolution:
If you are using manual configuration for Communicator, please check that the server name 
is typed correctly and in full.  If you are using automatic configuration, the network administrator 
will need to double-check the DNS A record configuration for OCS_Server_FQDN 
because it could not be resolved.

Log Name:      Application
Source:        Communicator
Event ID:      7
Task Category: None
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      <Client_machine_FQDN>
Description:
Communicator failed to connect to server OCS_Server_FQDN (IP_address) on 
port 5061 due to error 10061.  The server is not listening on the port in question, 
the service is not running on this machine, the service is not responsive, 
or network connectivity doesn't exist.
 
Resolution:
Please make sure that your workstation has network connectivity.  If you are using 
manual configuration, please double-check the configuration.  The network administrator 
should make sure that the service is running on port 5061 on server OCS_Server_FQDN (IP_address).