As the summary said, please get call-ui component work on the last Plasma, so folks could call each other. Reproducible: Always
You might want to try 16.04 release that has latest ktp-call-ui working on latest Plasma. Please report any bugs you might find as separate bugs. Thanks
I'm on Kubuntu 16.04, and there is no possibility to add a SIP account. And when I am in chat with xmpp account, the "call" buttons are grayed. XMPP server is Jabber.ru and they claim to have a possibility to call each other.
You need KDE Applications 16.04, regardless of Kubuntu version (yeah, bit confusing..)