Thanks Ella,
We have already tried entering the manual configuration sipdir.online.lync.comb but it resulted in the same error.
Note we also get the same error when trying the Lync client to one of our trial accounts also using the onmicrosoft.com domain, so not related to our domain sip records.
So;
We get a 'unidentified developer' error on install of the Lync 2013 client (downloaded from Microsoft Office365),
and then Lync wont connect with the error "Sign in to Microsoft Lync failed because the service is not available or you may not be connected to the internet".
The unidentified developer error usually means that either Microsoft signed it's code incorrectly (unlikely?), or that the certificate it used isn't recognised on an Apple mac for some reason (like a certificate authority being one that isn't trusted on a mac? How could I check this?).
Once we bypass the security warning about the "unidentified developer" the client still cant connect. Anybody able to advise whether this looks like two separate problems, or whether the mac has some security setting that stops the Lync client from connecting when it cant identify the developer? (we have the built in firewall in the mac turned off).
Any suggestions?
Ed.