I started this track.
Before I put my hands on practice scenarios, I like to get fundamentals first.
Please guide me which portion I should read /study first .
Do you have any previous Voice experience? If not, I would recommend starting with the CCNA Voice books such as the title from Cisco Press, and then also at least two of the CCVP books (Gateways and Gatekeepers and Troubleshooting IP Telephony). These books will provide a background for the theory and an introduction to the configuration process.
If you do have a theory background, then I would recommend starting with our CCIE Voice Advanced Technologies Class and the Volume I workbook, then moving on to the Volume II workbook.