Lync Online/Skype4BOnline - "The server is temporarily unavailable" and you cannot Sign In.

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You have been using Skype for Business Online for a long time and everything was working fine, but, someday you get to the Office and nobody can connect to Skype for Business Online anymore.
By what you know, this probably is an issue with Skype for Business Online, but you go to the Office 365 Portal and you can't see an alert from Microsoft.

Probably at this moment you already checked some Microsoft articles like:

How to troubleshoot sign-in issues in Skype for Business Online

https://support.microsoft.com/en-us/kb/2541980

"Cannot sign in because the server is temporarily unavailable" error when you try to sign in to Lync Online in Office 365 dedicated

https://support.microsoft.com/en-us/kb/2619522

However, you know that everything is fine in your environment, as it was working until yesterday.

So, at this moment you are already calling to Microsoft to open a ticket. Don't do that yet! This can be an issue in Microsoft side, but yet this can be something on your environment.
If you have a restrictive firewall, have you been following the RSS feed with the new IPs that Microsoft adds to the service? If not, check as below:

This is the oficial IP list that Microsoft updates from time to time:

Office 365 URLs and IP address ranges

https://support.office.com/en-us/article/Office-365-URLs-and-IP-address-ranges-8548a211-3fe7-47cb-abb1-355ea5aa88a2

If you access the article you can sign to the RSS feed: https://support.office.com/en-us/o365ip/rss and every time that the article is updated, you'll know and be able to updated your firewall rules as needed.

This is very important, because new IP's are added to Office 365 from time-to-time and if you have restrictions by IP instead of names, and  you do not keep up-to-date with this list, eventually you can start to have issues to Sign In Skype for Business Online.

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