On linux machine following settings needs to be done in order to get in browser calling enabled for phone numbers. Installation setup
1. Install twinkle setup (Create user profile) and get it working for outgoing and incoming calls
2. Install telify add-on (can be un-installed once completed with all settings)
3. Copy wrapper script twinkle_tel) click here to download and copy it to "/usr/bin" on user's machine (make sure it has executable permissions)
4. In Firefox go to "preferences/applications"
5. Search for "tel" protocol and change value of tel protocol to "/usr/bin/twinkle_tel"
Usage/ Pre-requisites
HTML source code of phone number should like as below

1. Dialing source code
<a title="phone number" class="telified" nr="9860648108" href="tel:9860648108">9860648108</a>
2. Call disconnect source code
<a title="disconnect call" class="telified" href="tel:disconnect">Disconnect</a>