What certificate do I need to install to get OCS to work on non-domain joined computer?

I've got a non-domain joined laptop that I need to get Office Communicator 2007 R2 working on.  The server is OCS 2007 R2 and the Laptop is Windows 8.1.  When I try to sign in to communicator I get the following error.

There was a problem verifying the certificate from the server. Please contact your administrator.

I am sure this is because there is a cert that I need to install that would normally be in place if the computer was on the domain.  

My questions is what certificate do I need to install on the laptop?  And will it be on the OCS server or on a domain controller?

Randy Knight, MCMPrincipal ConsultantAsked:
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David Johnson, CD, MVPOwnerCommented:
how many front end servers do you have?

    FQDN OCS pool = ocspool.company.local
    FQDN OCS Front End Servers
To correctly request certificates for both front servers enter the following -
    Certificate name = ocspool.company.local
    Certificate subject alternative names include -
        DNS Name=sip.company.local
        DNS Name=sip.company.com
        DNS Name=ocspool.company.com
        DNS Name=ocssrv01.company.local or ocssrv02.company.local
Jakob DigranesSenior ConsultantCommented:
Guessing that the certificate on OCS is enrolled from an internal PKI - you need to install the internal PKIs trusted root certificate on all non-domain PCs connecting to OCS (from LAN).

Log on to OCS server - go to certificates snap-in - computer certs. Find the OCS cert in use - go to certification path - select root certificate and click open/view - and then COPY TO FILE to export this certificate. Then import to COMPUTER STORE (important!) in Trusted Root Certification Authorities, on the non-domain PCS.

This makes your non-domain PC trust the certificate for the OCS server

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