I have recently joined an institution that uses Skype for Business as telephony solution. This is a commercial product from Microsoft that was never released for desktop Linux (and probably never will).
There is a Linux clone called Wync that is able to connect to the same telephony network, but this is just another commercial solution.
There is a plug-in for Pidgin called SIPE that is supposed to integrate with the Skype for Business network, but it seems nonoperational on Ubuntu 14.04. Once the plug-in is activated Pidgin keeps regurgitating error messages that render it useless.
What other free options are there to connect to this telephony network from Ubuntu?
There is some information here from Chris Rob about using Skype for Business on Ubuntu 14.04 with Pidgin.
Seems you need to compile/install the latest SIPE plug in for Pidgin and then configure to connect to the Skype for Business server.
I have made some, but not complete progress with the following.
For me, the standard pidgin sipe plugin, through Office Communicator, let me connect to Skype for Business and see my employer's user directory, but crucially, did not succeed in a test call or a call to another user, with a variety of error messages. Even text messages to others did not arrive, despite pidgin giving no error message. However, there is a newer version of the plugin, as indicated in this Reddit post:
To make it work, I did:
Now, the test call works. However, when trying to join a conference call I got: