Source code freely provided to you by Doubango Telecom ® under GPL v3 terms.

click2dial is an open source and free click-to-call service using SIP as signaling protocol and WebRTC as media stack. This service uses Doubango VoIP framework and webrtc2sip projects for inter-working with any SIP-legacy network or PSTN.
The goal is to allow any person receiving your mails, visiting your website, reading your twitts, watching your Facebook/Google+ profile to call you on your mobile phone with a single click.
As an example, click here to call me on my mobile phone.

WebSite: http://click2dial.org/