Hi Jsilven2,
This is Neo Zhu from Office365 support. Sorry for delayed the response. Based on my understanding, the users which located in “site-c” can’t do the authentication with ADFS server via Sign-in assistant.
The login authentication will do the following steps:
1, Login to Lync online service
2, redirect back for ADFS authentication
3, use the local DNS and find the ADFS internal IP address
4, connect to ADFS and use DC for the authentication
5, Pass / login
During this time, sign-in assistant will handle all the authentication process. I also would like to check the uccapi log via office.microsoft.com/.../troubleshooting-sign-in-issues-HA101859258.aspx . You can upload that log via the workspace.
Thanks,
Neo Zhu