I've been having this issue for quite some time as well, and have been working with a Microsoft Skype for Business (SfB) support engineer on it. To be certain it's the same, or similar-enough issue, here's the setup I've been working with (or against it seems):
Verify you have the latest version of Skype. Check your security software or Firewall settings to make sure they aren't blocking Skype. Are you using a VPN? If you are connected to a VPN and the call is unable to connect, then disconnect from the VPN and try the call again. Are you using a proxy server? Sometimes, we can’t connect to Skype as we are using a different server due to which there are some complications. Thus, the proxy settings on the device should be checked from time to time, as there might be a need to manually connect to Skype and log in again as the server was facing problems.
* Skype for Business 2016 (any version, including latest in the insider 'Fast Ring' releases)
* Macbook Pro - Early 2011
* OS X 'El Capitan' - v10.11.6
* Corporate domain is on Azure AD (Active Directory)
* When signing in with the AD account, you are either a) Given a choice between 'Work or school account' or 'Personal account), b) usually automatically redirected to the company branded signon page.
If that matches up, the next step is to check the certificate which is on domain's federation services (ADFS) host. This may require getting IT involved to find it, or reviewing the SfB logs.
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When you get the host name, for example, 'adfs.mycompany.com', go to that address via HTTPS in Safari. So you'd go to 'https://adfs.mycompany.com'. Once there, click on the lock to the left of the address in the address bar and click the 'Show Certificate' button. Make sure that the selected certificate, in the tree view at the top, is the bottom-most one. The bottom pane should show some brief certificate info (Issued by, Expires, 'This certificate is valid' type message, etc.). Expand the 'Details' section in that lower pane, and look for the 'Signature Algorithm' line, which should be, roughly, the 12th one down. If on that line, you have something similar to 'SHA-512 with RSA Encryption' (forget about the long number afterward), then that is the source of the issue with logging on, and also, activating Office 365 (if you have a company account for it).
OS X prior to 10.12 (Sierra) does not *natively* support 512 bit certificate signatures. So while browsers and everything else shows that, yes, the certificate is good, valid, unexpired, etc, the low level network stack in OSX, which is used by SfB to initially connect, does not, so it cannot validate that the certificate is valid, thus causing this issue.
Unfortunately, at this time, there doesn't appear to be a way to wedge in support for 512 algorithms in OSX, and that includes forcibly upgrading/linking openssl. The only way I've found to date, to use SfB on anything less than 10.12.x, is to essentially MITM yourself, using a proxy application, such as Charles, which will create its own fake certificate which you must trust, to connect.
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I apologize for such a long writeup, but given that despite my hours and hours and hours spent over months of researching the issue, I hope to provide as much useful and helpful information as possible for any future Googlers/Bingers/DuckDuckGoers/etc.
Skype For Business Mac Cannot Connect To Server
Jul 13, 2017 8:50 AM