Hi,
It seems that the federation setting on your side not set correctly.
Please check the federation configuration such as external policy, DNS records, PIC setting with the help of the link below:
https://support.office.com/en-us/article/Let-your-Skype-for-Business-users-communicate-with-external-Skype-for-Business-or-Skype-contacts-b414873a-0059-4cd5-aea1-e5d0857dbc94
If the issue persists, as the forum you posted is for SFB Server On Premise only. The issue is more related to SFB online, you can also post a question on Office365 forum for assistance. Thank you for your understanding.
http://community.office365.com/en-us/f/166.aspx
Best Regards
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Eason Huang
TechNet Community Support
