i understand your problem which you mention in your post. it may helps you.
When users of Microsoft Windows Messenger 5.1, Microsoft Windows Messenger 5.0, Microsoft Office Communicator 2005, or Microsoft Office Communicator 2007 try to sign in to Microsoft Office Live Communications Server 2005 or to Microsoft Office Live Communications Server 2003, they receive the following message:
Signing into SIP Communications Service failed because the service is temporarily unavailable. Please try again later.
This issue occurs if the name of the domain that Live Communications Server is configured to host does not match the domain part of the Session Initiation Protocol (SIP) Universal Resource Identifier (URI) that is assigned to the users. For example, this issue occurs if Live Communications Server is configured to host the mydomain.contoso.com domain, but instead the SIP URI references the contoso.com domain.
To resolve this issue, add the domain part for the SIP URI to the Domains tab in the User Services Global Settings Properties dialog box. To do this, follow these steps:
1: Start Live Communications Server.
2: Use one of the following methods, depending on the version of Live Communications Server that you are using:
In Live Communications Server 2005, right-click Forest, and then click Properties.
In Live Communications Server 2003, right-click User Services and Global Settings, and then click Properties.
3: Click the Domains tab, and then click Add.
4: Type the domain name that corresponds to the domain part of the SIP URI. For example, type contoso.com.
5: Click OK.
6: If the previous domain information is not part of any SIP URIs, remove it. To do this, click the domain, and then click Remove.
7: Click OK.
8: Restart Live Communications Server. To do this, follow these steps:
( i) Expand Servers, right-click the server, and then click Stop.
(ii) When the server has stopped, right-click it, and then click Start.
Microsoft Office Live Communications Server 2005 Enterprise Edition
Microsoft Office Live Communications Server 2005 Standard Edition
Microsoft Office Live Communications Server 2003
Microsoft Windows Messenger 5.1
Microsoft Windows Messenger 5.0
Microsoft Office Communicator 2007
Microsoft Office Communicator 2005