[UPDATE 1] I am aware of a similar thread but it does not address the problem of no contacts.
I followed the thread and discovered that the thread started was the only one to have that feature working properly. It is believed that whomever he calls, gets the same feature too but the opposite person needs to have Lollipop as well as the latest(2.11.508) apk too from the website of whatsapp. Other user reported that using terminator and root access, one can get the calling tab enabled but the issue is no one can see other contacts and my bet is that none of the contacts would have the latest apk as well as lollipop loaded up. So, I've got an idea through which we can enable the feature and help others get the feature too. Try to follow the given steps.
-Rooted Lollipop(Stock or Custom ROM would do)
-Latest apk from here
-Terminal from Playstore
-PM me to get my contact number and add me to your contacts(To test out the feature)
1) Make a Backup of your current chats if you want your chats to be maintained.
2)Uninstall your current version of Whatsapp (Necessary Step, do not try to update through intsalling the latest apk over current version)
3)Install the latest apk you downloaded.
4)Verify your number, restore your chats if you want and start whatsapp.
5)Close Whatsapp and remove it from recents too.
6)Start Terminal and type the following commands without double quotes:
"su" - At this point, it would ask for root permission. Allow it.
"am start -n com.whatsapp/com.whatsapp.HomeActivity" - This would start Whatsapp with 3 tabs namely Calls, Chats and Contacts.
7)Head on to contacts tab and message me from there. I would also add you to my contacts and see if I could call you or not and if it works, we all would get the feature permanently!
[UPDATE 2]It has been confirmed that this method does not work for now but at least keep your contacts list updated with someone having lollipop and following the same procedure so that you may come to know when you are also eligible to test out the feature.
Credit goes to pradnesh07 and Tropiux from Reddit.