[douche]

Skype for Business and Skype are two completely different, just barely compatible things. They run completely different networking protocols, and the S4B client is still, barring some cosmetic skinning, the same as the Lync client from nearly four years ago. The About page in the S4B client may still be calling itself the Microsoft Lync Client - it was still doing so in the 2016 version of the client this spring...

What a colossal marketing cockup

[ZenoArrow]

Very few people care about networking protocols, and SFB is explicitly a business communication tool, meaning that most of your contact will be with people who also have SFB. As for the About page of SFB, I can't comment on what it was before, but I've just checked, it now says Skype For Business 2016.

[douche]

I only know the details because I write software on top of some of the Lync/S4B APIs, and have to deal with customers that are completely confused why one Skype client they have for making calls works with our software, and their other Skype client with a nearly identical icon that they also use for making calls doesn't...

[ZenoArrow]

Which app do these users have? If they have SFB, does this not work?

https://support.office.com/en-gb/article/Let-users-add-exter...