This link from MsExchange provides details and screen snapshots for installing LCS.
And here are instructions from Ray & Damian specific for LCS with SBS2003:
1. Open LCS, right click User Services Global Settings, Properties. Under the Domains tab, click Add, and add the .com domain (i.e. domain.com)
2. Under LCS again, in the Users folder, verify the user is enabled, and the SIP URI is set to sip:firstname.lastname@example.org
3. Go to DNS, create a forward lookup zone called “domain.com”.
4. Right-click domain.com, Other New Records, select Service Location (SRV). For the Service, type _sip, for the Port number, type 5060, for the
host offering this service, type the FQDN of the server. (Like the attached picture)
5. Ipconfig/flushdns, ipconfig/registerdns.
6. Go to LCS, right click the server, select Stop, then Start.
7. Logon to the IM (Windows Messenger 5.0) using the username email@example.com under SIP Communications Service Accout.
As far as SharePoint, you need to go to http://companyweb, Site Settings, Go to Site Administration, Manage Site Collection Users. Update the email
address to @domain.com. Also, the client needs Office 2003 installed. If you don’t have Office 2003, Outlook 2003 is fine also.
But watch, any externally hosted website abovementioned ‘domain.com’ will become unreachable because of the new DNS settings. If that is the case:
You could just add the WWW A record to your locally hosted domain.com zone to point to the public IP of your externally hosted web site.
The other solution would be to host your internal LCS users using firstname.lastname@example.org.