VoIP free SDK for Web, Android and iOS for Video Calling and Chatting

I tried searching but with no avail. What is the easiest way to establish it? Requirement is I should be able to call or message from any plateform (iOS, Android or Web).

I did establish my XMPP/Jabber server for chatting (single and grouped) but I am unable to find any help for free video calling.

Some nice suggestions are needed, I can still change/trash all the things I have done yet.