What you need, what you have, and what you're actually using will tend to be different - e.g. in your case, as your VOIP traffic builds / increases, the data transfer will tend to increase, but a true picture of what you are actually using you will know within a few months ... and you can then get an idea of your "average" transfer on a monthly basis.
At the moment, your graph will simply show your max / peak usage ... depending on how the statistics software is configured / displaying the information.
At present, I would leave the line speeds at 20Mbps - see what your usage is after 1 - 2 months, and you can then decide what your NEED is acurately.
I was confuse before and you make myself in a puzzle
Can you plz tell me what is 20 Mbps Unmetered means
Can I get 20 Mbps upload and 20 Mbps download full duplex mode by using 20 Mbps Unmetered .
You should ask your provider that. If this was a direct order of bandwidth from a carrier the answer would be yes. However if this is a dedicated server then it could mean 20Mbps dedicated to your server, or 20Mbps circuit shared among a group of servers and usage is not metered. If you have 20Mbps dedicated to your server than yes, you can and will be able to use 20Mbps full duplex.
As for what you are seeing as already mentioned that is your actual usage. You should never see a bandwidth graph showing 20Mbps up and down, and if you do that would mean you need to upgrade.